What Causes Late Periods And Cramping?

Hi my period is usually regular but I'm 3 weeks late now (april 14th -last period), im only having cramping. I have taken 4 preganacy test (3 home test and 1 at the gyn office yesterday. My doctor performed a sonogram and ultrasound, and nothing is showing up. What is going on? Im going to have blood drawn, what can i expect? am i preganant?