The first time I got pregnant, I got a strong cramping one day, so I was sure that AF would arrive the next day. However, on the next day, all the cramping was gone, and there was no sign of AF. I waited and tested few days later, and got a BFP. I guess the cramping was due to the implantation.
I have heard that some cramping is normal. You should start worrying if you have cramping with bleeding.