I am 13 and I have this lump underneath my nipple as well, and it started out with just pain and very tender to touch, and now it formed into a lump that does not hurt anymore. I do not know what to do? What if it is Cancer?
If its even on both sides then its nothing to worry about. Theres always going to be lumps in your boobs because its your glands. As you get older then mature and get bigger which is why boobs grow and your body lays down fat in them too. You have nothing to worry about I had the same at your age. Im about to turn 20 and now know I was worried over nothing. If you want reassurance then see your doctor.
I have quite large feeling lumps in both of my breasts directly underneath my nipples. They're called growth buds. I got them at about 13 and still have them, even though I'm 18 now, but they will go! It is just a part of development. If you only have it on one side it's worth getting it checked out. Meanwhile, check for colour change in your nipples and I suggest you lay your fingertips flat over your breast and press firmly (not enough to make your breast sore) and move your hand in circles to check for any other lumps. You should do this right up to your armpit to check for swollen glands there as well. If you have small boobs don't confuse the feeling of your ribs for lumps. If you've started your period try not to do this check just before your period or while you're on as hormones can also make your breasts feel a bit lumpy too. I'm sorry this is a lot of information to take in! If you want extra reassurance talk to your mum/ a female relative, they were the same age once too! And if you're still worried you can always ask a parent to take you to see a GP (it's sad to say, but a lot of the time you'll be taken more seriously if you have an adult with you). Please try not to search into it too much on the internet as you'll find a lot of scary stories about worst case scenarios, in reality you are likely to be a very healthy, developing young girl!