Charlie, since breast tissue does extend up into your armpit, you might be feeling a small cyst - which would feel as you describe, moveable. Though truthfully, this sounds like an enlarged lymph gland. Lymph glands can become enlarged anytime you have an infection somewhere in your body, so it wouldn't necessarily be related to a breast issue, if that's what it is.

That said - we're not doctors here, and can't diagnose this. Any breast change or armpit lump lasting longer than a month should be checked by a doctor - period. Since you're 18, you need to get yourself in gear, emotionally, and get over your fear of seeing a doctor when you feel something's not quite right, health-wise. To put it bluntly, it's foolish; if by chance (and the odds are very low) this is something that needs treatment, the longer you wait, the worse it'll be. Why not put an end to your worry by simply seeing the doctor and getting it over with? Action trumps worry any day; think how good you'll feel once you get past this and can simply forget about it! So call the doctor, OK? Take care- PJH