I'm 2 weeks late and the sides of my breasts are killing me. i don't think i'm pregnant tho. breast cancer runs in my family, so i'm a little worried.

Very rare.: Breast cancer at your age is very rare unless you have a hereditary predisposition (more than 1 immediate family member with breast / ovarian CA , a known BRCA mutation or breast cancer at a very young age). Breast cancer presenting as pain in both breasts would be distinctly unusual. If you have been having unprotected sex I would be more inclined to consider pregnancy.

Late period: The most common cause of a missed period in a sexually active patient is pregnancy - check a home pregnancy test to be safe. Breast cancer is unlikely in a patient of your age.