Chelsea, this article on breast pain will give you some background information about types of breast pain and help you decide if you need to see the doctor.  Tender breasts can also be an early sign of pregnancy, so if there is a possibility that you might be pregnant, see a doctor right away.  

Although at 18, you are near the end of puberty, you might also want to read our Teen Guide to Normal Breast Development.  If that lump on the right side of your breast has been there longer than a month, then you need to let a doctor check it out.  Lumps in teenagers are almost never cancer, but sometimes they need to be removed and checked out just in case.  Depending on your medical history and other drugs you might be taking, it would probably be OK for you to take an over-the-counter pain medication like Tylenol when the pain is the worst.  Talk that over with your mom or the doctor though before you take anything on a daily basis.