Hog levels decreasing


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.