My DD(7) did this at the same time, too. She was teething, and her bottom two teeth came in very quickly. I had to break her quickly, so as much as it hurt me to do so, every time she did it, I took her off the breast, thumped her mouth, and sternly said "NO!" She had to wait about a minute before I'd let her back on (usually to the other side if the bit one hurt too much). I stayed on advil/tylenol for a few days for soreness, and she settled down, and learned to teethe on other things (paci, teether, clean wet washcloths, etc.) I'm not a lactation consultant, but I've nursed 3 to around the 2 year mark (youngest still going at 22 mos), and I'll send you a PM with my number so you can call if you need additional support! :)
Good Luck!!