Oddly, my first thought was that it was a staph cyst that had gone unnoticed and drained overnight. I've never had one, but a friend of mine did, in almost that same spot, and it left a massive (for lack of a better word) crater in her skin. She had no indication it was there until the night before, when it itched a little, no redness. She thought it was a bug bite. By morning it had left an unspeakably nasty mess in her sheets and turned into an enormous scab, after which it healed up on its own without further intervention (which was good, because she had no health insurance).
Apparently cleanliness isn't the issue, it's just a Thing That Happens.
If it was a cyst, though, I can see where it would be particularly traumatic for poor Hanners. Staph are bacteria, and she's (at least) OCD and germaphobic. This might set off a whole series of self-destructive neuroses she previously had under control
The Staph thing would certainly match up with Jeph's comment about gross pictures, as they can turn quite nasty.