Hi, if you're 18, you can go to a Planned Parenthood clinic and get an exam. (It's not only for birth control; you can get a regular ob/gyn exan there.) Probably the bump is nothing, and if you were to say to your mom that you need to see a doctor because you have a urinary tract infection or something else that sounds innocuous, she wouldn't raise a fuss or think you have been having sex. But if you don't want to chance it, see if you can get an appointment at Planned Parenthood. I assume you are using protection when having sex?