M.F.
The same thing happened to me about the same time! I ended up going to the Milk Bank at TX Children's Hospital to get some lactation help. Turns out with me, my son's latch had all of a sudden changed and he was really pulling on my nipple, causing the pain. The more he fed, the more it hurt. And I was so upset, because I didn't want to stop nursing, but I was practically crying every time I had to feed him! They helped me with the latch and we had to go through a little "re-training" period, but after about 1 week, we were back on the right track with no more pain for Mom!
I recommend really paying attention to how he's latching on and if he doesn't latch on correctly, gently remove the breast and start over. He'll get the clue that if he does it "wrong", he won't get to eat.
It's funny - you've been doing this for so long now, it's just become second nature. But if we're not still paying attention to what's going on, bad habits can form and things like this happen!
Good luck and keep up the great work. You are doing the best thing for your child!