I've been on bcp since I was diagnosed 7 years ago, so hopefully I can help a little bit.
For me, regardless of bcp or not, I always stall or gain a couple pounds the day before or day of my period. It usually stays that way until it's finished, and then I'll lose at least a pound plus any I gained. I know that everyone is different, but I still have PMS and retain water
Good luck to you, I'm sure as time goes on you'll figure out your body's pattern!