Confused. Any help?

I am EBF my 6 month old daughter and got what I thought was my first period since delivery. I called my OBGYN to set up an apt for birth control. She asked about my period and I gave details. Well apparently she thought something was wrong because I was/am bleeding extremely heavy for the last 4 days. She told me go to ER. I did. The ER doctor said the exact same thing. I shouldn't be bleeding this much. Started me on iv, did blood work, pelvic exam, ultrasound. When ultrasound came back he asked me if I was pregnant I said no. He said they found a small sac in my uterus but don't know what it is. My blood/urine came back negative for pregnancy so any advice on what this could be?