Sore nipples at 16mo pp

My son has been breastfeeding for 16 months. At first, it was painful, and then it went to barely feeling like anything except a slight tugging when he nurses. So, since he was maybe a month old, it hasn't felt like much.

Now, all of a sudden, since a couple weeks ago when my son turned 16 months old, my nipples are sore again! He still nurses a lot, too! Never less than 4 times, always around 5-8 times a day.

I don't think I'm losing my supply, because if I pump, I still get some decent ounces. He isn't showing any signs of wanting to ween, which I'm fine with. I'd like to make it to at least 2yo.

So why the sore nipples? It's so weird!