Hi, M.-- You are a trooper! Nursing in a variety of positions is smart, to help keep milk flowing through all the ducts. Let me just remind you that ibuprofen is compatible with breastfeeding and may help you stay more comfortable while you heal.
This sore could be a blister from Baby sliding off the nipple and slurping it like a straw instead of coming at it with a big open mouth, but since you are confident in your latch, I doubt that's the trouble.
It could also be thrush, but thrush often hurts all the time, not just with nursing, and the entire affected area is red, shiny, or covered with tiny blister-like sores. Sometimes it is white and has a mother-of-pearl sheen and also looks like a milky coating inside the baby's mouth. Moms describe it as feeling like an awful sunburn on their nipples. It needs to be seen by your health care provider if this is it, since it's hard to kick without a prescription treatment.
But what I think this is, is a "bleb." Here's the information on blebs from the Breastfeeding Answer Book, which is La Leche League's medical text:
"White spots on the nipple (also called "blebs" or milk blisters) have been observed for as long as women have been breastfeeding but have not yet been fully explained. Some think they may be caused by a plug, such as a granule of thickened milk blocking the milk flow near the opening of the nipple. Some think they may be caused by a thin layer of skin blocking the opening of a milk duct from the outside. In 'The Breastfeeding Atlas,' Wilson-Clay and Hoover observe that sometimes mothers notice a bleb after the baby has bitten the nipple and suggest that they may be a wound over which a callus has formed. Ruth Lawrence, MD, suggests that it may be a small pressure cyst formed at the end of the milk duct.
Many have observed that these white spots sometimes coincide with bouts of mastitis, but the cause and effect is unclear. Either the bleb itself causes the mastitis by blocking the flow of milk from the duct or perhaps the bleb forms as a result of the mastitis, which caused the milk to thicken in a blocked duct.
"If the bleb is painful, consider applying wet heat, either with warm compresses or by soaking the nipple in warm water. Another approach is to wear a cotton ball soaked in olive oil in your bra to soften the skin and then attempt to peel away the thickened layer of skin. Then, attempt to express from the nipple any remaining material by compressing the nipple behind the plug. Sometimes it is possible to express a thickened string of milk, which will help open the duct and keep it open.
"If the above treatment does not bring quick relief, consider going to your health care professional to open the blister. (If you try to open it yourself, it may result in an infection.) In some cases, opening the blister will cause the milk from behind it to start flowing and bring relief. In other cases, the bleb will be dry. If so, continue the previous treatment to help express it out until you get some relief.
"Once the blister has opened, take steps to prevent infection. . . avoid strong smelling soaps which may casue baby to reject the breast. . . .consider applying a thin layer of topical antibiotic ointment sucha s Bactroban or Polysporin after feedings alternated with ultra-purified lanolin, such as Lansinoh to prevent the formation of more thickened skin. Because the antibiotic ointment is only minimally absorbed into the skin, almost none passes into the milk. . . If you continue to experience recurring blebs and/or plugged ducts, some suggest reducing or eliminating saturated fats in the diet and taking a lecithin supplement."
Mastitis is a generic name for any infection taht settles in the breast tissue. You can avoid it by nursing often and having excellent self-care--good food, resting, etc.
I hope this helps! I am sorry nursing is hard right now--this is just a speed bump and you will be enjoying your snuggly time with you baby again soon. I notice you are in Syracuse--I am in Ogden and if you have other breastfeeding questions I might be able to help you with, you can find my number on the La Leche League Web site at www.llli.org. All phone help and other services are completely free.
Best wishes!