Thanks all for your replies.
After a few days of being in a funk I am feeling better.
I guess going bad to work just triggered some old anxieties- I was hungover every single day during the 8+ years I worked at my current job. I had severe panic the last few days I worked last October and I felt like that again on Monday when I went back there.
My workplace is a very negative, depressing atmosphere full of miserable people - most of them have been doing the same boring job for 30+ years. We have been really slow the last few years, nobody has gotten a raise in years, and everybody worries about getting laid off. Moral is at an all time low.
I still haven't been back to work since I was there last Monday- my boss told me to take all the time I need to get better - we are so slow at work they can easily get along without me.