The last couple of days I felt like a blister was forming in the middle of the ball of my foot, but couldn't see anything. Today it was particularly painful at work, so during lunch I went into the bathroom to see if I could finally get the darn thing. I'd just removed my shoe and sock, got my foot on my knee, flexed it as I tried to see what the heck was going on.
Well, it "popped" and fluid went squirting. Thankfully onto the floor in the empty stall next to me and not my face or clothing. It's still sore, but feels much better now.
No clue what caused it either.