What's going on with my metabolism?

So, for the past 6 or so months, I've been eating like 600-700 calories and running like 30-40 minutes a day. Needless, to say, I've lost a ton of weight, too much actually. I kinda figured I'd know when to stop, but I guess I didn't. I just want to stop caring.

I decided I'd make myself eat 1200 calories a day and then increase that amount until I found the number of calories over which if I ate I'd gain weight. Only, I've gained like 2 pounds eating 1200 calories in 2 weeks. And, okay, so 2 pounds really isn't that much, but its a lot for 2 weeks. Why is it that eating 1200 calories makes me gain weight? Have I thrown off my metabolism that much?

Your body has adjusted to 600-700 calories per day over that 6 months, now you suddenly doubled your calorie intake.  Your body needs time to adjust to changes, I don't think you can permanently damage your metabolism because the human body is so adaptable (survival mechanism), however its now adapted to survive off of 600-700cals per day.  600-700cals a day is not very easy to live off however, so its probably saving all the calories it can get expected 'starvation' again soon. 
I'm facing the same problem as you to a much much lesser extent however.  What I've been doing is slowly increasing my calorie intake, about 100 calories every week or so, while keeping my exercise routine the same and staying active, it will give you time to adjust to the increase.  Also if you gradually increase your calorie intake and have any sort of strength training in your exercise routine then any weight you put on will be much more likely to become muscle (which in itself takes more calories to maintain).
Anyway take it slow and keep your weight tracked so you can watch what happens over a long period of time, and remember that weight your gaining isn't necessarily going to be pure body fat.

So should I increase slower? Like start at 800?

From what i've read I think its best to increase slowly.
Also, i'm not sure about you, but doubling my calorie intake all at once would be tough and probably leave me feeling sick and bloated.
Also if you increase slowly your not going to gain 5 pounds in a month since you can watch and see the effects of increased calories slowly.  Your weight will increase when you increase calories initially anyway, its your body's natural response, but if you keep doing it your body will realize your not going to starve yourself again, and so will stop storing calories, start using them, and increase your metabolism.

Since you already increased your intake to 1200, I would suggest just staying there.  You've gained two pounds (probably, yes, you guessed it, water weight).  And since you're running that long everyday it could also be muscle in your legs.  Next time you shave, see if you notice any increases of muscle in your legs.  You can also feel how well your clothes fit.

You already went up to 1200, you're not going to gain more than five pounds at the most.  If five pounds is too much, try something else besides cutting calories or exercise to help you lose weight (ex. drinking more water).

i used to eat 200-500 cals a day.  then i uped it to 1050, and at first i gained, actually alot.  but now that my metaboism is sorting itself out im losing again. 
stay strong girl, your doing the right thing by eating more.

bettybooya,

were you excercising when you increased the cals suddenley from 200-500/day to 1050?

thanks