Found a small painful enlarged lymph node near my breast/under my armpit. apart from things like cancer what else could it be?

See a doc: Could be an infection, a cyst, but cancer is the one you need to see a physician in person to make sure.

Infection: Infection anywhere in the hand and arm and result in an enlarged painful lymph-node in the armpit.

Axillary lumps: Cancer would not be the first diagnosis we would think of, there are many other benign causes such as adenitis, inflamation/infection od the sweat glands, abcesses. See your doctor for a diagnis. In the meantime do some warm compresses. It may help.

Inflamation: Lymph nodes respond to inflamation in the area that they "drain." So the most likely cause in a young woman is an infection, though fortunately rare, it could be a cancer. Please see you doctor if it does not resolve over the next few days.

Also consider: that it may not be a lymph node, as you could have a cyst or small abscess in this area, or other nodular lesion,