Sorry, I meant I have a IUD, not a uti. Haha my mistake
Well, first off, it is possible (though not likely) to get pregnant when you have an IUD, so you could take a home test just to be sure. But you'd almost certainly have breast sensitivity, not just nipple sensitivity, if you were.
Sometimes the creases inside the nipples (there are lots of them, it's where the milk comes out when you're nursing) can get full of dead skin cells and feel sore because the pores are blocked. You can scrub them in the bathtub with a slightly abrasive glove or sponge, or even use the blunt end of a (cardboard) Q-tip stick to more or less pick them out. It sounds weird but in a way it's like they get pasted shut, and it can hurt.
Can't think of any other suggestions. Hope these help.