I have found a marble sized lump in my left boob.. close to the armpit but not close to the skin. i am 22 what could this be?

Cyst: It may be a benign cyst but you want to make sure it's not something more serious so see a doctor.

Many possibilities: Fortunately, at the age of 22, breast cancer is fairly rare. With that in mind, there are a number of benign causes of a breast lump, including cysts, fibroadenoma, fibrocystic breasts, lipoma, &infection. Your doctor will likely take these possibilities, along with your family history or other risk factors, into consideration when you see him/her. Which you should do, just to be safe.