Sore Nipples???

I was on birth control for 4 years. My husband and I have been trying to conceive our first since January. I’ve had blood work done, internal ultrasounds, the whole 9 yards. I’m healthy, my uterus is good, my ovaries are good. 2 and a half weeks ago I was put on Metformin to regulate myself period and ovulation wise after being on birth control for so long. About 3 or 4 days ago my nipples became sore to touch, wash, or even put clothes on. I am at a loss as to why?? 🙃