Ultrasound Worries Update

So this morning I went for the ultrasound I told u guys I was worried about bcus the OB said baby girl was under weight for 38 weeks & 4 days. But she's 6IBS 4OZ. They said my fluid is on the line of being low. I have to have another ultrasound Monday & a non stress test before my apt & do kick counts every 2 hrs until then. 10 kicks every 2 hours. What does non stress tests do & does it hurt? I'll be 39 weeks exactly Monday when they do it. She's due the 14th. Any advice. Will I have to be induced if I fail that test? I'm a first time mom.