Threatened Miscarriage

A little back story, at 8 dpo I had a positive hpt. Started taking progesterone. By 13dpo when my period was due I started having brown discharge which eventually turned to dark red. I bled for three days and light to spotting on the fourth. Went to the er yesterday (which was the fourth day of bleeding). They stated that they saw a gestational sac (I’m 4weeks at this point, if pregnant) hcg came back negative. Took a hpt yesterday and it came back with a vfp. Now bleeding has stopped but wouldn’t the sac be expelled if I was miscarrying. I’m just trying to understand. The hospital stated that it was a threatened miscarriage and a chemical. So why is the gestational sac still in the uterus? Please help.