R. -
I've BTDT - twice! When you feel the pain - do you notice that your nipples turn white and then red?? It could be a form of Raynaud's syndrome - blood vessels constrict and restrict circulation in extremities. (And warm compresses are an excellent idea!) I have it in my hands and feet - and breasts when nursing.
I'll bet your daughter is going through a growth spurt - and she may have changed her nursing position and/or latch technique. (Both my kiddos became "lazy" at about 4-5 months. It was like starting all over again!! ARRRGH!!)
If you've never attended La Leche or another support group - then I highly suggest the Breastfeeding Support Group at Olathe Medical Center. It's a good way to learn from other moms. Brenda - the nurse lactation consultant is WONDERFUL!! She can usually pinpoint any problem within minutes. The meetings are on the 2nd & 4th Wednesdays of each month. (I still have some very good friends that I met through that group six years ago.)
Good luck and keep me updated on what you find out.