Sometimes a low-fat diet isn't the best way to go to lose weight, just FYI...have you kept a record of what you're eating, how it affects your stomach, etc.? Sometimes this can show patterns that you never knew existed. And most young women have psychological issues about their weight anyway, so you really have to look critically at the issue: are you considered a normal weight for your height? If you are in your late-teens, early-twenties, a little weight gain is nothing to be concerned about.
I am not sure if you've had your thyroid checked but sometimes hypothyroidism can cause some weight gain, in addition to constipation issues, so that might be something to check out. I am sorry you are not feeling comfortable right now, and I'm sorry if you felt neglected on this board (it's been a little slow lately)! Don't worry, we're still here for you!
Co-moderator - IBS Forum