I also had the same experience and it gone in few days without any medication. It is not breast cancer and I suggest you not to poke into such bumps with needles as it may lead to septicaemia.
WHAT!!! DO NOT just start poking needle's into yourself, if it was cancerous the last thing you would want to do damage the tumor causing the cancer cells to spread to other areas of the body. I know your only fifteen but come on; why would you think to do that?
Now on to the logical explanation of what your lump is, more than likely the 'lump' your referring to is an enlarged lymph node. You have over 600 lymph nodes all over the front of your body, With the majority found on the torso from the groin area to behind the ears.The function of lymph nodes is to hold and release immune cells to fight local infection. So if you had a swollen lymph node in your arm pit; that could mean you have an ear infection, local abrasion or bruise; or simply the lymph node was irritated causing it to swell.
If you have something that is of serious concern to you; regarding your health it's important that you see a doctor for an evaluation. It's great that your paying attention to your body so that you can notice differences when the occur, but don't just jump to the worst conclusion when something seems unusual.
So... now that you have injured yourself by stabbing a needle into the lump I think it is very important the you have it looked at by a doctor soon.
You might want to get it checked out just to be sure In the mean time look for any more symptoms or if you just don't feel good write them down in a journal and show them to your doctor and ask them there opinion.
Hope this helps