Freaking out..

Came to the ER because I was having some cramping. I had a few positive home pregnancy tests so I was so worried something was wrong which is why I came. Urine negative, blood showed that I am pregnant but the doctor said my hcg count was “low” did ultrasound showed nothing. Said that the radiologist couldn’t rule out a fetus being there?? I’m so lost. Any advice? Also to add I’m pretty early in the pregnant maybe 4 weeks?