What Does Missed Period Indicate?

I have taken 4 pregnancy tests and they all say I'm not pregnant. But I haven't had my period for over a month. I never miss my period. But every great once in awhile, I get cramps and my facec breaks out like I am getting my period. I don't know what to do. I really don't want to go to the Dr.s office, because my parents will see it on their bill. What's wrong with me?