You're entitled to your feelings, and I can appreciate it's hard to feel like the maternal grandmother l knows more information. I would try to remember that your son is not the patient and he is not the one carrying baby. My guess is your son is trying to keep you in the loop, but his wife is anxious (who wouldn't be) and he doesn't want well-meaning expressions of concern or support from you to be misinterpreted as an invasion of medical privacy. How would you feel if your son told his wife all about your yeast infection and she reached out to see how your discharge was? You are, after all, the grandmother of her child and she has a vested interest in your health. However routine pregnancy may seem, and even though we've all been through it, this is still pretty intimate and women do have a right to their medical privacy. Her uterus doesn't become community property just because she is pregnant.
Yes, this child will be your grandchild, but DIL isn't a mere grandchild vessel. She is this child's mother and she is the patient. I think it's understandable that she would like to choose what she does and doesn't share about her own medical information. Especially when there is nothing to be done by the other parties involved.
If it helps ease worry at all, a repeat scan to check growth and baby/placenta position at 36 weeks is completely typical in my next of the woods (US). It's not a cause for concern.