Hog levels decreasing

Mia

15 week pregnant went in to ER with extreme lower back cramping, they checked my hcg levels and they were low compared to the 13th week that I went in for bleeding. This Dr seemed concerned so he performed an U/S, there was a heartbeat and movement from baby but yet he told me to contact my obgn right away, I thought hcg levels are supposed to decrease during a certain point in pregnancy? I’m scared and confused, had 2 previous miscarriages in past.