can your body reject the pill

I've been on the pill since March this year my body was fine with the pill then in July I gad surgery and ever since my body just hasn't been the same.. It like before my surgery had my periods normal like once ever three to four weeks and they lasted for a week and a bit but since the surgery it's like I'm bleeding every second week and it won't stop for a couple weeks and shit...what should I do? Should I change pill or what? Help !!