Haven't had sex but no period?

So I was away from my S/O for about 3 months, but during that time he came to visit. When he was visiting, we had sex twice, I think one time was with a condom and one time was without. After those 2 times, I got my period, and now I've completely missed my November period and I'm PMSing now but still havent gotten anything? I know I'm not pregnant because my S/O never finished inside me or anything. My periods are irregular, and I have completely missed a month before but that was due to a medication I was on and I've stopped that medication now, so do you think it's just my body being weird? Or could it be something serious?