Why do you think you're 5w2d? Is a that based on your last period? If so, you may be quite a bit less than 5w. I think it's terrible that doctors rush women in for "early ultrasounds" - there's often nothing to see because it's far too early and then the mum just gets stressed.
Placenta doesn't form till sometime in the 5th week, towards the end I think. Might be a bit too early to see anything. Cramps without bleeding are pretty common, especially for a first time Mom. No need to worry unless there is bleeding. If your HCG is good then don't worry until your next ultrasound if nothing shows there.
When I was 3weeks I started "cramping" but it was just stretching my body was trying to make room for what was coming I didn't have to have an ultrasound but I'm now 32weeks 3days with a healthy baby girl so I hope that's all it is for you!
Has this happened to anyone and then they saw a baby?