SORE NIPPLE DURING PERIOD 😩

So I got my first heavy PP period (I say heavy because I had one at 5.5 months and it wasn't bad at all) a few days ago around 7.5 months. 

I don't think my supply has been affected, however, my right nipple is extremely sore and  is slightly painful when baby eats from that side. My nipples never hurt, not even when we were starting out. My son has always had a great latch and been a fairly effective nurser. I don't see any visible ailments, so I'm wondering if it's just a hormone thing? Has anyone else experienced something similar? How long did it last and does it happen every month?

Thanks in advance!