Feeling confused and disappointed

On February 15 I took a pregnancy test and it was positive. My SO and I had been trying for almost a year to get pregnant so we were ecstatic when the test was positive. I called my doctor and told her the news. My first prenatal appointment was today. Before scheduling my appt I told them I didn't think I'd be 7 weeks by today but they told me they had to go by my last period. So today I go in just for NP to tell me that there is a 50/50 chance I will miscarry. WTF? Again, I told her I didn't quite think I was in the 7 week time frame yet but she fuckin' insisted that they go on my last period. Now she wants to see me next week to see if my jelly bean has grown at all. I am very dissapointment on how my first appointment went. I'm stressed out thinking the worse but trying to stay positive and have faith in GOD that the NP is just an idiot and that my calculations are the correct ones. I think I know my body and according to my calculations I'm 5 weeks and 1 day. Just trying to stay positive and hoping my jelly bean will stick. Please tell me if anyone has had a similar experience.