Your IBS acting up near your period is pretty common, myself included. As for the weight gain, have you been to a endo? If your TSH comes back in the normal range then have them test your antibodies. My TSH was in the normal range yet I was putting weight on and so tired all the time. My antibodies were sky high and I learned I have Hashimoto's Disease. Your immune system attacks your throid gland. Sounds like me, you have more than one thing going on. Good Luck!