I have the same thing you have mentioned. I went to my doctor and he said its nothing to worry about and it is supposed to go away over time. I have had it for almost 4 years though and it hasnt changed a bit.
Hi, i am also an 18 year old male and i have a lump on my left breast between my nipple and armpit...although there was no pain, there was a slight discomfort. the lump is totally moveable.
I was at the doctors getting a check up on my throat and i mentioned it to my doctor. he checked it out and told me that it was a something on my tissue(cant remember the name), and because it was moveable, i had nothing to worry about - especially at my age. He told me the only reason i would need to get it removed is for cosmetic reasons, if it got too big. But my point is, to be honest, if i wasnt going to get a check up, i probably would have never went, i was just a bit scared, and still am...probably because im a hyper-condriac.
Anyway, go get it checked up, although it probably isnt breast cancer, it could be a benign tumour that needs removed.
Oh, and i still have it, and have had it since about october.
Though it is true that males also develop breast cancer what you describe and considering your age this may well be related to the changing hormone levels in your body. You might consider seeing your family Dr. for an examination and explanation of this problem. These symptoms occur quite often in males in your age group.