please help

Amanda
Hi ladies! 
I have been successfully breast feeding for 5 month old. First month was full of the usual painful nipples, cracking, readjusting patch etc. everything has been very easy and smooth since. The past 4 days my left nipple has been increasingly starting to hurt when he nurses. It's a very deep, throbbing pain. Nipple is swollen and a little red. No blood or white. Baby doesn't seem to have any symptoms of thrush and is latching and nursing on it as usual. It is just extremely painful for me. Any ideas on what could be going on ? Thanks in advance!