West Highland White Terror- Oops.. Terrier

Ok, so here she is... The love of my life, and today a thorn in my side. My beautiful Westie. She is sweet, gentle and wonderful w/kids (a huge plus for a teacher). However, lately, as in the past two weeks, she has been marking my place like crazy. I steam vaccuumed the carpets and it seems that since then, she's been marking. I found a spot or two and have cleaned them, but I could still smell a faint scent. So, this morning I got on my hands and knees and sniffed. Yes... I looked very much like the dog, while she sat on the rug and watched. Guess what I found- probably 20 spots!!! She is lucky that I love her so much. I cleaned them up, but had to get ready for work. In the crate she went, b/c she isn't getting free reign today after all of that. Does anyone know what might be causing this? She's 11, and I've had her 3 years. We've never had this problem before. I'm wondering if I should take her to the vet, I'm just dreading a huge bill. I think I'll do a little internet research.