Fetal assessment

Hello everyone, today I had my regular appointment I’m 36w and 4 days, my OB told me my results for my Group Strep B came back negative I was happy, she measured my belly it looked to be on point, then.. she checked to see which position the baby was laying by feeling right above my pelvis ( she was feeling for baby’s head she’s done that since 28 weeks and baby was head down) well today she couldn’t feel the head and she was uncertain of how baby was laying, she set up an appointment for a “Fetal Assessment ultrasound” I’m totally freaked out and i guess I’m looking to see if other moms have had this done and what should i expect what do they look for?

I’m a FTM and I’m just super worried and having anxiety since this morning.