Milk supply low.. Advice.

Hi, I'm currently breastfeeding my 4 month old & also formula feeding. I've been doing that since he was born. Before I had alot of breastmilk I feel like & now I feel like I might not be producing much because I can breastfeed my baby & he seems to not be full so I have to give him a formula bottle & then he'll be happy. I also just recently got surgery to get the Mirena iud removed because it was misplaced so I don't know if maybe the medication I received from my stay at the hospital has anything to do with my decrease in milk. Now I'm thinking I should just stop breastfeeding but i feel bad to just not try anymore.