Get some Daktarin (or maybe it is Daktozin) nappy cream from the chemist to clear it up - it's an anti-fungal nappy rash cream and will get rid of the rash if it is thrush. My DD had some thrush and this cleared it up.

I have a real mental blank with the name of that cream, I can never remember the name but I know it when I see it! It will be there with all the others (Bepanthen, Desitin etc etc).

Also...I'm still on my different teat/sippy cup theory...my friend's DD suddenly rejected her bottles about this age and only wanted to drink from the really cheap cartoon character bottles you get at the supermarket. Just a thought...

ETA: If she is eating cheese she'll be getting plenty of calcium. A matchbox sized lump of cheese is the equivalent to 250mls of milk.