There are lots of reasons that this could be happening, and all we can really do is guess at what those reasons are at this point.
Personally, I think some kind of anxiety might be likely.
For anyone who doesn't have anxiety problems, it's very difficult to understand what it is like. I absolutely hate staying in other people's houses, even family members who I know quite well. It's just out of my comfort zone, playing by other people's house rules, having to make conversation, and there's little escape. I don't stay at people's houses for this reason. If ever we are going somewhere, I will make sure that I am able to stay at a hotel.
A lot of people will think that this is silly, but if I was this woman right now, stuck upstairs in someone else's house, I simply would not be able to make myself walk downstairs and make conversation, not without the support of my husband anyway.
I'm not saying that's definitely what's going on, just that you shouldn't assume that someone is rude because they're staying upstairs. Anxiety can be absolutely crippling, and add that to jet lag and exhaustion and I just can't even imagine it. You've said that you don't know her well, and for someone with severe anxiety, being in someone's house who you don't know well - particularly when your husband has gone out for the day - is just an idea of a nightmare!
Of course, that might not be the case.. but it is a possibility.