4 days late and still getting BFN

I was supposed to get my AF on the 14th and still nothing. I've had cramps on Monday and Tuesday and I've been a little nauseous. Also I've been super gassy. But I felt for my cervix and it's pretty low but soft. I tested to days before AF was supposed to come and got a BFN and then again Wednesday morning. My periods always regular. What do you thinks going in? Has this happened to you?