Increase in appetite - a possible side effect, but not definite.
Weight gain - only possible if you don't control your diet if you end up with an increased appetite.
I'm sorry, but I don't see why you wouldn't even be willing to try something that could make you feel better, especially since the weight gain isn't that likely. I'd much rather try something that has a good chance of improving my PMS and periods, than to completely ignore it because there's a slight chance that it could increase my appetite. As long as you control your diet, and if required, do a bit of exercise then you won't get that negative effect.