I'm so late but I'm not pregnant

Okay so my boyfriend and I had sex about a month ago and when he was taking off the condom I was still on top of him so everything just kind of spilled out. He freaked and I was just like "nothing happened because that would be so impossible" fast forward about 2 weeks after when I'm supposed to get my period and he brings over two pregnancy tests. I took them both over a period of two days and they were both negative. It's been 3 weeks since that and I still haven't gotten my period. I took a test yesterday because my mom was concerned but it was still negative. Why am I so late??