Hi vnmum
Teething will make bottoms really sore because all the excess saliva reacts with stomach acid in some chemically way I don't understand, but it makes for evil nappies.
What are you using for cleaning? I was using just water and washable wipes, but on my HVs advice (I have the only good HV in the Western World!) she suggested that that woudlnt' get tid of all the poo traces because it can be quite waxy. So I've made up my own invented solution which is working wonders (dd's bum would regularly bleed with rash until changing this.)
I mix a generous squirt of that Simple baby lotion - no perfume/colour etc, some drops of tea tree oil and boiling water in an old ice cream tub and then soak the wipes in it. Flannels would work fine too. It's gentle enough not to irritate, but has enough power to shift all of the eurgh on the skin. Better than conventional wipes too.
Regular baths - maybe even morning and evening? (Hassle I know but might work)
Metanium in a thin layer is also very good.
Use fleece liners instead of flushable ones.
Silk liners are meant to be good for nappy rash although I've never tried them.
Definitely go and see GP and mention thrush - don't wait for him/her to dismiss you.
I wouldn't put Canesten on your ds' bum without advice cos it can be quite powerful stuff.
Give all your nappies as extra rinse in the wash just to make sure the washing powder isn't irritating already sore skin.
Um....can't think of anything else, but will keep pondering!