Sore nipples after period

As the title states, my nipples have been extremely sore since before my period and now after. I was worried I was pregnant before my period but took 4 tests, all negative, and then I got my period. This period was a little weirder than normal. The first day it was heavy bright blood and I passed a lot of clots then it got light and then it got heavy then it got really light again. I figured this was from the plan b’s I took in the past couple of months. Anyway, my period ended yesterday but my nipples are still sore. What is going on?