why am I gaining so much weight?

I am on the pill, since I've been on holiday this year, seems like I have lost my way and I feel like I'm gaining so many weight And I am having my period like normal, I am getting super tired so early lately. In between taking my pills my boobs aches and my stomach. I'm in a lot of stress with uni work aswell, which doesn't help at all! Can someone please tell me what is happening to my body?