Yep or if your periods are really regular and she knows it, maybe complain how heavy and painful they are and you want to regulate it. I'd definitely suggest still using condoms to avoid scares even if you do get on the pill.
I don't know how laws are where you are, but in California, you can see the doctor alone if you are 12 years of age or older without parental knowledge. There is also always that one point where the doctor pulls you aside that you can discuss the issue with your doctor and see if he or she can help you come up with a reason to tell your mom and prescribe you some.
This sounds awful, but if you don't want to go into detail, tell her your periods have been really irregular and its worrying you. Because it's not healthy to have too few or even too many periods.