BFN but no period?

My last period wasJuly 20-26, I had sex on the 10th. My period was lighter than usual, but it was also heavier at some points. 
​Since then, I've been nauseous all day for weeks, tired, extremely moody, I've had cramps above my pelvis and in my lower back. I've eaten a lot more than I've ever eaten in my life, and my jeans are starting to be really right fitting.  
​But when I took some tests one was a BFN and the other was an electronic one and just gave me a "?"