It's probably a hematoma (bruise) on the inside of the nipple from the bite. It can take a while to heal. Imagine hitting a soft part of your body hard, it can take a few days for the bruise to show and resolve. Consider using a nipple shield while it heals to prevent additional injury.
Also, make sure that he latches correctly. I was super sore when my daughter got her teeth, because she started latching kind of lazy. That was also the only time when she would bite me (when she was almost done and just sort of hanging on). So I started correcting her latch or taking her off the boob every time she got lazy - I was never bitten again and the soreness went away.
Good luck.