If you are underweight or even on the low-end of normal, then you do not need to be worried about gaining some weight, as long as it is not very rapid weight gain (not more than a pound a week, I'd think). I am concerned that you say that you are a low weight, then say that you are fat. I've never had an out-and-out eating/body disorder, but I did come close at one point in time while dieting. The more weight I lost, the more unhappy I was with the way I looked and the fatter I thought I was and the more weight I wanted to lose. One day I realized that I really hated my body and I had not hated my body like that when I was a good deal larger. Needless to say, the dieting stopped.
If you are abnormally proportioned--like you look like you are very pregnant--then that is often a gas problem. Talk to your chemist and see what he recommends as treatment for gas. I know there are products in the UK that will help with the gas. You might also try probiotics; they can cause additional gas to start with, but after a week or two, they usually stop doing that, and many people have found that they end up reducing a good deal of the gas and help give more regular bowel movements too.