Just got my first bill

I just got my first bill from my doctors appointment. I went in two weeks ago  because I was bleeding. They did some blood work and a 1 minute transvaginal ultrasound to make sure the baby was in the uterus. My insurance only covered $50. My bill is $300 and I have another appointment on Monday. I'm thinking about canceling. I thought my insurance covered 40%. I'm freaking out. They didn't even cover my office visit! I don't know what to do.