No Period??

I don't know what it is, but the past couple of months my period has been a little weird. I'm sexually active, but only since May and this first happened in April. My period was 14 days late in April, basically spilling  over into May and then I got my May period later that month. June, July and August came just fine but my September period is 19 days late and counting. Like I said, I'm sexually active, but we use protection and I've already taken a few pregnancy tests just to be cautious, but they're all negative. I'm going to make an appointment for the gyno, but still, what are other potential causes for me to miss my period besides pregnancy?