It is not terribly uncommon for IBSers to gain weight (especially with IBS-C). IBS causes many of us to bloat. Lots of members have talked about how they look pregnant and can't fit into their clothes.
Here is a link to a post about weight gain:
IBS-C WEIGHT GAIN
Are you on only medication that has weight gain as a side effect? I remember when I got on birth control I gained like 10 lbs. Same goes for steriods, my buddy got a severe case of poison oak and the doc prescribed Prednisone -- he gained almost 10 lbs from being on it for 10 days.