What should I do?

Sometimes my period changes but I was told it was normal when you're under 21, but I rarely miss my period. I have only missed my period about two years ago, I am scared that my period won't come this month. What should I do? Take a pregnancy test? But at the same time I don't think I should take one because my boyfriend and I barely have sex it's like every three months or something. Last time I had sex was two days ago and I didn't get my period yet. Also I did receive my period last month but didn't have sex at all. Please help me.