Hcg levels

I went to the hospital today because I had pain on the right side of my abdomen and back! They did an ultrasound to see if they can see the sack in the uterus but really no luck the dr said he “thinks” he can see something in the uterus but he isn’t sure if it’s the baby! And also told me that my hcg levels were low but it could be since I’m only 4 wks 5days and to check with my obgyn within 2-3 days. I am seriously so scared! Has this happen to anyone?