Hi I've been breastfeeding for over 2 years no...

Hi I've been breastfeeding for over 2 years now (and tandem feeding DD1 and DD2 now)but for the last few days one boob has been really sore. My attachment looks right and I am quite certain its not mastitis or a blocked duct. This one boob hurts non stop and when I feed and have a let down it really kills. The nipple itself is red but I can't see how I've hurt it. Any ideas?