I have a big red spot on my left breast that came up on Sunday and got worse. Having traipsed round the internet looking for answers, I'm pretty sure it's a boil caused by infected pore blockage caused by my new habit of kickboxing and waiting too long to shower afterwards. (There had been another few red spots before this one that went away faster. This must just be the first one that got infected.) It's quite flat, though - it obviously has pus in it quite deep under the skin but it doesn't have a head, really. I've covered it with germolene and a plaster.
Thought you'd like to know, you bunch of cats.