RH shot?

Firstly, how come I am not definate on my blood type? lol. Anyways, I'm 7 weeks and 4 days and I'll see my doc again for an ultrasound on Monday (at 8 weeks). The Nurse Practioner wants me to come in earlier to get my type tested to see if I need the shot. I'm thinking I should be fine until Monday ( I can't keep taking off now and need to save my days for when I really need them). What are you all's experience with this shot? How soon did you get it, etc.?