What is a STAT ultrasound & why would you require one?

Im 36 weeks pregnant and I went for my checkup yesterday. The Dr. told me I had a large amount of white blood cells in my urine and gave me antibiotics. She said she was going to order another ultrasound to be done but gave me a referral to go have it done at the hospital. So I went in today and asked them if I needed to make an appointment for it and they said since it was a stat i didn't. so i went where the radiology department was and they told me since there were 4 people waiting to have a stat ultrasound i was going to have to make an appointment so they gave me one for the following wednesday. That left me wondering what is a stat ultrasound and why would you need to have one done at this far along the pregnancy since I already had my last ultrasound when i was 7 months and i thought they only do 2 throughout the whole pregnancy. I didn't sk this question to my Dr because i thought it was just to check the positioning of the baby. So my question is what is a stat ultrasound and why would i need to have it done this far along?