Gaining

I have to agree with some of the comments/advice. I have gained only around fifteen pounds. I have been a jogger for many years and still jog, even though I love gaining weight. I too have felt the difference in my fitness since gaining. I admire how much you gained and how fast, but I think you should see where your body weight ends up by backing off a bit.
I'm quite old, so my gain isn't out of the ordinary for me in my age group, in fact it is virtually expected. Given your youth, I would take a break and see where your body adjusts.

I think you should probably try to start doing a bit of exercise, as some others have suggested.

I could definitely benefit from trying to get back in line with an exercise plan. I used to do yoga for an hour every morning, but my momentum got interrupted when the channel dropped my yoga program. I've got some DVD's now but haven't been able to make myself use them.

Anyway, I digress. Just sounds like you need to take a little time and try to get acclimated to your new weight. Then, after you've built some fitness, reassess whether or not you want to continue getting fatter.

Add walking to your daily routine. I know that to many that is a "SIN" but it is important to take care of yourself. Walking helps keep you strong, and is low impact to prevent excess wear and tear. Also, swimming.
By staying strong and "fit" underneath, you can get big and fat on top! Can't build a monument on a poor foundation!