I just wanted to add an update in case anyone with a similar problem finds this thread. It's been a week since the pain started, and it's almost healed now. I found some info on kellymom.com that suggested that it was from a bite. It said to soak in salt water for 1 minute after each feeding, then cover with ebm and let air dry, then cover with lanolin, and to wear loose fitting shirts with no bra when possible. I did that Saturday, Sunday, and Monday, and by Tuesday morning it was much better. I was also applying antibiotic ointment (neosporin) at night after the DF and wiping it off before her first morning feed. Today (Thursday) it's almost completely healed, but still hurts a bit when she's nursing.
Thanks for all your help!!