Help!!! I didn't get my period last month

Daniela
So.. I had a "boyfriend" until the first week of October, we had sex a couple of times but he always used a condom, I cheated on him with a friend and he didn't use a condom but he never finished. Long story short I didn't get my period last month, I've taken 2 pregnancy test and they were both negative. Any advice ? Could it just be stress ?