Reason For Missed Period Without Intercourse

I have a question about my stomach. I haven't had my period in three months and I never had sex in my life. My stomach keeps getting bigger even when I don't eat much and my whole family is skinny and thin. It's just my stomach. How do I get rid of this? I am naturally thin as well and I exercise on my stomach like crazy. Why isn't my stomach flat like it used to be?