My sister has breast fed all 4 of her children and has experienced the same thing. Your hormones are (obviously) going nuts after you're pregnant and especially while breast feeding. I would go to a fam doc to have your hormone levels checked because your pituitary gland is what controls lactation and other bodily cycles. If you were to have them checked and find that your hormone levels are a little off you might be able to do something to balance them out.

Also, like Phrick said, breast feeding can cause build up of milk and little lumps. Sometimes there can be an infection in the breast that is regularly used for breast feeding.

Oddly enough I learned this because of my cat haha. Ever since she was a kitten she has sucked on her own nipple. The vet told me to keep an eye on it because it could cause an infection in her nipples. And when I checked it out, apparently the same thing can happen to humans!

Don't ever feel grossed out or self-conscious about it. We're pretty open here!