To exercise or not to exercise?

I have been counting calories diligently for four months, and have not gained anything. In fact, during the first month I lost seven more pounds. I know my calorie intake has been wildly fluctuating - sometimes I manage 3500+ (and still don't seem to gain), other times I've only been between 1200-1400. For three months now I've stayed at the exact same weight without gaining or losing, regardless of how much I do or don't eat.

In any case, my Mom keeps telling me that if I don't exercise, that when I do gain, I'm going to gain the weight back as fat. And when I've told her that no matter how much I eat I'm not gaining, she says it may not be because I'm not gaining per se, but because I'm losing muscle from lack of exercise and replacing it with fat from the food I'm eating, therefore my weight is staying the same even though I'm actually gaining fat. I don't think that's happening because I'm just super thin and I can grab hunks of loose hanging skin that feel like there's no fat there, but the thought still scares me. I'm not afraid of gaining weight, but I want to gain it back as muscle, not fat. I'm kind of terrified of gaining it back as fat, I'm a former athlete (body builder and sprinter). So besides walking and occasional situps, I go on the exercise bike every day and bike 1-2 km. (I don't have enough energy or muscle to bike any farther - I mean I could push myself, but I'm scared to injure my atrophied muscles). That only burns about 20 calories, so I don't think it's anything major. But I worry - is exercise going to stop me from gaining weight, if I'm not even gaining on the calories I take in anyway?

Also, my heart has gotten badly out of shape. Back in my athletic days, I could not push my heart rate over 120 even after a hundred yard dash, because I just had an extremely fit heart (resting pulse of 64). Now, even walking up the stairs can spike my heart rate up to 130. My Mom tells me that in order to get my cardiovascular fit, I need to do cardio on the exercise bike - raising my heart rate and keeping it there (she wants me to aim for spiking my heart rate for a good 20 minutes eventually) and then gradually letting it come back down. Now, as a former athlete, I know this is good sound advice for an athlete wanting to increase their cardiovascular efficiency, strength, and endurance. I'm just worried it's the wrong thing to do right now - I actually worry I might give myself a heart attack, becuase I just don't know what my heart can handle right now. I don't think I've damaged it during my long period of hardly eating, but I don't know. I definitely hate the fact that my heart pounds like crazy after the slightest exercise, and I want to get my heart fit again, but I'm just scared to push it too hard right now. Maybe I'm worrying too much?

I'm sorry, I know this question is getting really long, but one last thing: back in my body building days, I was an obsessive exerciser. This was not ED related. You have to understand bodybuilders - exercise and gaining muscle is our whole life, it consumes our thoughts. Besides weightlifting in the gym every day, and jogging and sprinting and hill running, I literally exercised almost every hour I was awake. I did situps every spare moment, even in bed at night. I turned every single action into an exercise - pulling milk out of the fridge turned into doing bicep curls with the jug. Walking up the stairs turned into calf raises and quad squats. I'd hold onto the edges of the counter or the stair banisters and lift my legs off the floor and do hardcore gymnastic ab exercises constantly. I drove my family nuts. My concern now is this - every time I start exercising, the old obsessiveness kicks in and I start doing more, and more, and more. Earlier this year I made myself an exercise program and my muscles were so atrophied that I damaged my tendons and muscles doing it. So now I try to be really careful, but the more I get on the exercise bike, the more I push myself, and I'm worried the old bodybuilding obsessiveness is going to start kicking in again, and I'm worried that might be detrimental right now. I don't know how to know when exercise is too much.

So, the three questions are: should I exercise at all (is this counteracting my attempts to weight gain)? Should I do cardio? And how do I stop my exercise from becoming obsessive?

 ... wow, that's not very helpful of your mum. Realise, though, she is a bit right. You can't gain all muscle and not gain some fat.

Given your other post saying you're struggling, and all the medical complications you have described, I think it is a bad idea for you to exercise. I would go to your doctor and ask if they can help you set up a physiotherapy routine, monitored, to help rebuild where your muscles have atrophied. I did this as I suffered pretty bad atrophy in my leg muscles. I still have moments where I fall over because they're weakened and a problem to stand on.

Cardio is detrimental to recovery gainers full stop unless they are willing to eat it back, but again given your complaints I do not think it is wise for you to do it. With former overexercisers, sometimes the only way to realise how important exercise is for health - not for cals burnt - you have to stop completely and then reintroduce slowly with assistance from a professional. Your heart rate is likely spiking not due to being unfit but due to your low weight and damage/stress your overexercising may have put on it. Again, this is something you need to speak to your doctor about.

Your mom sounds like she's being counterproductive.  There are a lot of myths that circulate in the general pool of thought and I suspect those are what she's accepting as facts.

I am a big fan of exercise, but I think you need to get your doctor's approval given your medical issues before exercising.  When you do exercise, only light cardio, just some mild walking for a maximum of 30 minutes per day unless your doctor tells you otherwise.  Weight lifting would be my recommendation to gain more muscle, but first you have to have a surplus of calories, without that the weight lifting is just burning up the energy that your body needs to survive.

Once you do start exercising, you need to plan your exercise and only do that amount.  No more, no less.  It's exactly like meal planning, but for exercise, calculate what you need to do and how much you can do given the amount of calories you're eating.  I'd recommend consulting your doctor about any exercise program that you start.