I suggest you have a response that you can go to automatically.
I love Lynn2000's suggestions that include "I don't want to go into the details."
There's also, "I hate talking about this stuff, it's really boring."
But you might need something more terse and more direct, like maybe, "I don't really want to share. Please don't push."
And practice saying it out loud a few times, so it will come more easily.
You could also brace yourself and have a conversation in which you say, "MIL, I really don't want to answer questions anymore about my health or my body. I'm finding that I'm really uncomfortable with that, so please don't ask me anymore."
If she argues with you, say: "Nevertheless. This is my preference, please respect me by honoring it." And every little argument she makes, answer with "Nevertheless."
Later, when she forgets, you can say, "I think you forgot--I don't like for you to ask me questions about medical things."
Here's the thing about being worried about causing tension: Tension already exists.
You are tense. So it's her turn to be upset.