It's a Monday disguised as Tuesday

I took Monday off. I have vacation time that I'm in a "use it or lose it" situation. Which is great, really, because now I have real motivation to take some days off in the coming weeks. However, I walked into the office today after a lovely day off and was immediately plunged into a horrible mood. It's not even like I was met with anything bad. I just don't want to be here.