Hi, I'm Pregnant And I'm Rh Negative Which Caused My

Hi, I'm pregnant and I'm rh negative which caused my last pregnancy to be a miscarriage.they gave me the shot that has the antigens or whatever in it after th I miscarriage and it lasts for 12 weeks. I found out I'm pregnant again and my shot is almost up, but they said not to get it till I start bleeding again. Should I just go to the hospital and ask for the shot? I don't want to risk another miscarriage.