When to maintain... Help please!

I'm new here. I'm 19 years old and I have been doing weight watchers for almost four months. I've lost 26 pounds so far. I now weigh 122 and I'm 5'7". I still feel like I want to lose a few more pounds, but I know that I should stop soon because this site tells me that I'm underweight, even though I don't think that I am. The problem is that I'm terrified that as soon as I start eating more, all the weight will come back. Am I being paranoid or can this really happen? How should I go about adding more calories? I also have another question. I'm only eating about 900 or 1000 calories a day. I know this might be bad but i'm only allowed 19 points on weight watchers and that's the amount of calories that those points add up to. Is this okay?

If you're only eating 900 cals per day, it seems very likely that you'll gain some weight back. You shouldn't go below 1200 calories/day.  Also, I don't think you need to lose any weight; if you're not happy now are you really going to be happy after losing a few more pounds?

Echoing what skinnyogi said, you should not go below 1200 calories a day. 

Your BMI is currently 19.1 so I would recommend not losing anymore weight and begin transitioning into maintenance mode.

If you add too many calories right away, you might gain a couple of pounds while your body is adjusting... Especially given the fact that you were slightly undereating to begin with.

To maintain your weight, you need 1700 calories (sedentary level) to 2400 calories (very active level). 

I would recommend first starting off this week by reaching at least 1200 calories every day.  Then next week work on adding 100-200 calories.  Continue doing this each week until you reach at least 1700 calories (your maintenance calories).