That was my thought too, perhaps you haven't waited long enough for implantation.
I have just gone through exactly what you are experiencing!!! I felt for about 8days the cramping I usually get when my period has started, plus I felt like I was a bit hungover, and I had tingling and shooting pains in my nipples and breasts... My AF was due today, I couldn't wait and tested yesterday and got a very faint positive. BUT a week prior, I tested and got a negative. There is hope yet matey!