Hi.
I would not be too concerned with the negative reading. I did a HPT on the day my period was due and got a neg too. I then tested 5 days later and BFP was on the horizon.
My cramping also started pretty early and has been constant through the last 5 weeks. My girlfriend who is a natropath says that it is just the uterus adjusting and stretching to make room.
Hoping you get a BFP real soon. Fingers, toes and legs crossed.
Mrs G