How do you mean sore?
Very early on it's all to do with your boobs gfetting to know how much to make to meet babys demands. Then it's possible to get mastitis so important to always try and make sure baby empties one boob before starting on other.
Bbay can also fuss a bitand make nipple sore. And then if bbay sleeps through a feed and boobs are full and not being emptied they can feel tender and uncomfortable.
My dd2 is 19 weeks and I'm breastfeeding her and generally no, no soreness but sometimes uncomforatble when DH and I are 'together' iykwim and sometimes uncomfortable in evening when DD2 can go through a stage of fussing and playing around and not feeding properly.