Advice please

So 4 weeks ago I found out I was pregnant. I then started bleeding and went to the hospital. I would of just going on 4 weeks and they did a internal scan. He said he saw nothing and at the end he spotting like a black thing? He took pictures and out them into my file. Their pregnancy test came up negative and he instantly turned around ur not pregnant. My question is should I retest what could this black thing of been??