I was 8cm in a similar position! I dilated very slowly and virtually without pain. I started dilating at 32weeks. I was 3-4cm for awhile, 5cm at 37 weeks and at 38 weeks 6cm contracting irregularly.
At 8cm, it didn't make sense to send me home- but I was also full term. So they broke my waters and my daughter was born 30 minutes later.
That beig said- is her due date may 13? Or does her third trimester start on may 13?
If she isn't greater than 36weeks they will keep her to ensure baby is as healthy and ready as possible for labour and delivery. If she's over 36 weeks I can't imagine why they would keep you any longer.
If she's early, it's preterm labor and it could be due to a variety Of things that your doctor could talk to you more about.