This is classic Post Partum Hemorrhage.
You want to replace volume loss with isotonic fluids(normal saline) while arranging for urgent grouping and crossmatch to replete volume in 3:1 of fluid : blood if necessary. You also want to do a pelvic exam to massage the uterus, remove any remnants of the placenta that might be present,give and oxytocin drip to stimulate the uterus to contract, repair any lacerations to the cervix or other parts of the uterus
Worst case scenarios you need to embolise the uterine arteries, do a B-Lynch suture and if all fails a hysterectomy.