Blood clot?

Amanda
I had my 12 week first look ultrasound today and baby looked great but they informed me that I have a small blood clot. It's 3cm. Has anyone else experienced this? 
She told me that I could start bleeding and pass it, it could absorb and disappear or it can just hang out in their until baby is born. 
There was no way telling if it's old or new. She told me not to worry but I can't help but worry. I just had a miscarriage from following this pregnancy so I pray this doesn't cause problems.