The pill.

I'm about to start the pill. I'm 14 years old and obviously not doing it for the birth control side, just to regulate my periods and sort some things out while I'm doing GCSEs. I was just wondering what the side effects are, I'm quite naturally slim and I'm slightly worried that it might make me gain weight and I'm happy and confident at my body now. Tyyy. Please no negative/hateful comments about me going on it, is what the doctors said for me to be able to have some control over it. X