I think ione has the same thing on her face, on both cheeks. I'm assuming it's a bad reaction to either a sponge I used the other day that had old soap on it or a sleeping bag i put her in that hadn't been washed before.
I showed it to the nursery nurse but she didn't seem too bothered by it, said it happens and a process of elimination will figure out what it is. I really hope it disappears soon though!