What Causes Missed Period While Sexually Inactive?

Well Im a female and Im 19 years old. I have missed my last two periods and there is no way I could be pregnant. I havent has intercourse in about 7 months. My periods have never been regular but I also have never skipped more than a month. When I do have my periods they are so bad that I cant even get out of bed, I scream and I cry and I throw up everytime. My mom and my grandma had endometriois, but my doctor told me that I do not. Im not on birth control, I havent been stressed until today when I realized that its been 2 months. Im not over or under weight. Im not doing dramatic exercises either. I dont know if this has anything to do with my cycle but I my sex drive is way down. Not normal for a girl my age. Im really scared about this so I hope someone can help me.