pregnancy/period

The last time I had sex was the 15th of December. I had my normal period the 4-9 of January (which is when my period is normally) my period should have started yesterday, and it hasn't. I guess I'm just paranoid, that I could be pregnant. I'm a nursing student so I'm already stressed enough, but this is stressing me out beyond belief. There's no way I can be pregnant if I had my period last month right?