You would think I'd remember this after nursing C for almost 20 months, but what are reasons that just one side would hurt, a lot, when baby nurses?

G is almost 3 weeks old and we had some issues BF early on, but her latch has always been good and things have been going great for a week at least. But yesterday the right side started really hurting when she latches. Like even using a nipple shield or changing positions, I can't tolerate her nursing on that side this morning. What should I look for? Clogged ducts? Anything else it could be? We've worked very hard to get here (oversupply, extreme and painful engorgement, fore milk hind milk imbalance). But OW. Even putting a bra and nursing pad on feels very tender.