CRAMPING *Ouch*

Melanie
I am happy to see my baby looking healthy and moving at 13weeks 4 days. Prior to this one, I miscarried at 1 week.  
I am in the hospital right now due to heavy cramping. Doctors were concerned because they were unable to find a heartbeat  But baby still looks fine and heartbeat sounds strong through ultrasound.
Anyone know why I have such painful cramps? No bleeding or spotting.  Tests are being done.