Weight Gain!

I"m a 47 yr old who was put on this medication to regulate my often-crazy cycle, reduce the incidence of ovarian cysts and possibly find relief from my Irritable Bowel Syndrome symptoms (my doc believes reproductive organs play a part in IBS). I'm in my 6th week of the medication and I've already gained more than 12 lbs (on my normal 105 lb frame) and increased my bra size to the point where I need new ones. I have also noticed a decrease in my libido (sex drive) and that's not good considering it wasn't all that strong before :( My moods seem pretty consistent. No worse than they were before. While I'm pleased to say I have noticed a definite improvement in the IBS symptoms, which were sometimes quite debilitating, I'm not at all happy about the weight gain. I have never been this heavy - even when pregnant full-term. And the bigger breasts mean I can't wear a lot of my favorite tops :( they feel so heavy and full.