What Causes A Delayed Period With Negative Pregnancy Test?

Hello, about a month ago, I had unprotected sex which was about 3 days after my period. Now this month, I ve missed my period by 3 days so far. Over this past month, my stomach is constantly cramping, I urinate more often then what I used too. I am always tired. I took a test last night which came back negative? But I was told that it couldn t be acurate because you are supposed to use morning urine.... Please help?