Help! House trained dog wee'd upstairs

[Cats49]

My 12 month old cavalier/bichon frise cross is house trained and never wees or poos in the house downstairs or in her crate. We let her come upstairs occasionally and no accidents normally but on one occasion when we didn't put her in her crate when we were out (DH couldn't be bothered to put her in it!) she went upstairs and wee'd and poo'd in our bedroom. And today she was allowed upstairs when I was sorting clothes out with the kids and wee'd on the carpet in our bedroom. She always asks to go out by standing at the door downstairs. Is it to do with the fact that we do not have carpets downstairs? She went to meet my next door neighbours dog a couple of months ago and after saying hello to their new dog, promptly went into their house through the patio doors and wee'd on their carpet. She has also wee'd on my mum's carpet once. Does she think carpets are similar to grass or something? ? It doesn't happen regularly but I would like to solve the problem! I feel nervous if we go to other people's houses with carpets!
Many thanks - sorry for the long post.