What Causes Delayed Periods With Negative Pregnancy Test Result?

My last menstrual cycle was June 22nd to the 27th.. I had unprotected sex a couple of times after that the last time was July 15th 5 days before my expected period.. I m 11 days late and I ve taken two tests and both negative... and just realizing I didn t get a period at all in July and its now August 1st..? what should I do?