What Causes Severe Cramping After A Still Birth?

I had a still birth on october 29th i was 18 weeks. His cord was around his neck twice. I went to the hospital tonight because i was cramping really bad. On the discharge papers the gave me the said i was having a threated abotion (light bleeding during pregnancy), Am I still pregnant? I took a home test when i got home from the hospital and it said negitive. I have had unprotected sex twice since my still birth.