By all means take her to the dr. to find out for sure. It might not be anything, but at 2 yrs old, my daughter had that also. At that time, the dr said it was milk gland ducts getting a little stopped up. I didn't understand that then, why would her milk gland ducts be doing anything at that age was my question? But he assured me it was common thing with little girls. It did go away with hot compress treatments. Then at 9 yr. it came back again. I wasn't as scared that time, but took her to the dr. anyway to be sure. It was very sore to her, and the dr. said it was her hormones starting to fluctuate. Not to worry and use hot compresses again. It went away that time too. He also told me, usually if it was sore it was not too much to worry about, but obviously keep an eye on it. So try not to worry too much, but get it checked out by a professional. I'd rather be safe than sorry, but most likely it won't be anything to worry about. Keep us posted what you find out.