I am 24 yrs. old and have been taking Toprol XL 25 MG at night fo...

I am 24 yrs. old and have been taking Toprol XL 25 MG at night for three weeks now, doctor prescribed for Palpitations and irregular heartbeat, also have Mitrovalve Prolapse. The problem is weight gain, and I have not changed my eating habits. If anything, I have been exercising more, eating about the same. (Also started herbal supplements about a week ago, pretty harmless, just vitamins/sage/etc.) and I STILL cannot lose any weight and feel like I'm actually GAINING. Has anyone else experienced this on such a small dose? I also feel like the toprol has affected my sleeping, however I was having problems sleeping before I started taking it (something that the doctor hasn't gotten around to addressing).