Yup, happened to my oldest boy off and on for a while a few years ago. We never did determine the cause. His would just come and go, usually at night time (terribly inconvenient when I am trying to put him to bed).
I have seen some kids have some really crazy reactions to viral illnesses. One of my dc kids last year got one that looked insanely like a chicken pox rash, but wasn't. And a friend of ours has a little girl who just turned one that caught a cold and then out of the blue had blister-like rashes, mostly on her arms and legs. Eventually, both got over them.