Have you heard of Raynaud's phenomenon?
I suffered from Raynaud's for years, without realising what it was. I can recall one occasion when I was standing in London waiting for a coach, feverish with the start of flu and with nipples that burned so intensely that all I could do was walk round and round in circles with my arms wrapped around my chest to distract myself from the pain.
It's caused by cold, you see. Many are the times I have been found kneeling on the floor with my chest pressed up to a radiator in an effort to warm my breasts up and stop my nipples from screaming at me. I remember my grandad showing me his white, bloodless fingers when he went out in the cold, and that was also Raynaud's.
The Wikipedia entry for Raynaud's phenomenon says that it can cause discolouration of the "fingers, toes and other extremities". I guess my nipples come under "other extremities". It is caused by constriction of the blood vessels delivering oxygen to the affected part, and can be triggered by stress, cold and... you guessed it... breastfeeding. If you're sensitive to strange images of nipples, then look away now. But here, courtesy of Better Health's Weird Medical Problem of the Week, comes a picture of the bizarre colour changes that occur when a nipple undergoes an attack of Reynaud's:

If you're wondering, that's white, then cold, cold blue, then finally red as the blood flow returns to the nipple.
It seems that some women experience Raynaud's during and/or after breastfeeding. It can be confused with the pain caused by a poor latch, so may often go undiagnosed. I have been one of the lucky ones; I have not experienced the condition since giving birth, though it had worsened during my pregnancy. It could be that childbirth and breastfeeding have cured it, or it could return in the future. Maybe I'm not suffering because we are heating the house more generously than we did pre-baby.
Raynaud's probably affects loads of people, but it is not serious enough to warrant investigation or treatment and hence it remains fairly unknown. Medications to relieve it are available, but knowing what causes it and avoiding the triggers are probably the most sensible courses of action.
Has your breastfeeding been affected by any bizarre disorders? I hope not, but leave a comment if it has! And if you've not experienced Raynaud's, I hope you enjoyed the picture anyway!