What Causes Delayed Periods?

Hi, I am 23 days late for my period. Im worried I may be pregnant. I had protected sex on April 2, but the condom slipped off, then I got my last period 2 days later, on April 4. I had protected sex again on April 10, and was expecting my period the first week of May, it still hasn't arrived. I have taken 4 pregnancy tests, all negative. I am worried that even though I got my period after the condom slipping off, it could still be a possibility that I am pregnant.