Can my boss demand a medical excuse to give me days off that she already approved?

I requested this coming Monday, Tuesday, and Wednesday off work three weeks ago. The time off was approved both verbally and in written form. Now my boss is saying she needs me in on Monday and Tuesday for important meetings. When I responded that I was sorry, but had already made plans, she asked me if my plans were related to a medical condition. I was taken aback by this question, because it seems like a major intrusion into what has now been categorized as my own time, and into a particularly private part of my time at that. Is it really acceptable for her to ask this of me? I mean not just in a legal sense – I’m pretty sure I’d have no leg to stand on in that arena – but also in the sense of professional appropriateness. I’ve just never been in a situation like this before, and don’t quite know how to take it. What do you guys think the best response to this would be?