it hurts!

So my son will be 3 weeks old on Monday, since he was born I've been breastfeeding and giving him formula. I never had problems with my boobs hurting so bad when giving it to him, just one day when my milk came in and that was it. Today I spend a couple hours without breastfeeding him due to being around a lot of family and running some errands. My left boob started hurting a lot so I thought it was because of all the milk that I needed to let out. I come home and feed my baby, after an hour the pain still doesn't go away. I put a hot towel on top of it while I'm breastfeeding and that felt so good. I left my son on that boob for an hour and it still hurts. The bottom right or my left boob hurts when I lay down, and even walk cause they move. I don't know what it is:( has anyone experience this??