not sure whats going on?

I'm not sure what to do im on day 36 of my second cycle on metformin and letrozole and I am ovulating my last cycle was 33 days. I have been having cramps and a feeling a bit of nausea for part of the week thought forsure I was going to start my period. On the morning that I excpected it to start i took a hpt but it was negative. If I am not pregnant where is my period? Does anyone know about this? Can a cycle on the these meds vary in length? I'm thinking of testing again tomorrow morning if its still negative i m thinking of calling the clinic I dont know what to do next.