From week 5 to week 9 I had cramps all the time. Implantation and your baby going in to your uterus causes it, that's what my doc told me..I'm now 24 weeks && he is perfectly healthy :)
It could be rounding ligament stretching feels the same. I had them real bad 1 night around 8w and bleed heaps but no 20w and fine. If your really worried just go ED