really bad nappy rash for the 1st time

[Elf1981]

My dd is nearly a year old and has hardly ever had nappy rash, perhaps once or twice. Though today, it just seems to have exploded, it's terrible. She's been whinging about sitting down or being held on my hip, screaming and crying immediately if she's had a wee or poo.
I've just changed her, smothered her in cream and she's asleep on the bed (was knackered, not sure if she'll be awake again in an hour or down til the morning now).
She was a bit whingy yesterday as well but no where near as sore. The only changes in her diet have been follow on milk in addion to usual EBM while I'm at work, but she's had that for 2 weeks now, and she had melon yesterday & today - could either of those be the cause?
Whats the best course of action? Do I wake her up later to change her? Do I give her a bath tonight? Do you just slop the cream on & leave it or rub it in (sorry for the dumb questions but we have never used cream before and she's started screaming when we've put cream on today)
TIA

two recommendations
chamomile tea, it is very soothing to sore bums, use it wash and bathe her bum in
also metanium cream - the absolute best, use sparingly to thinly cover the rash
What have you been using thus far?
if she normally eats melon then unlikely to be that, its pretty alkaline

and nappy off time!

Let her sit in a bath of just water - no soap at all. Pat her dry gently with asoft towel and leave her bare-bummed as much as poss. The best cream we have found is Tui Bee Balme - look for it in health food /natural product shops. It's really good on lips too.

I had planned to put her without a nappy when we got home today, but she was knackered and fell asleep on her highchair so have brought her to bed instead.
Melon - she's had it once before, and again this week. I didn't think melon would be a problem.
I had a little pot of Bepanthen ointment, shall have to invest in metanium, thanks!

metanium cream is the best nappy rash cream I have ever used.

Ds3 has nappy rash that bleed before, with big blisters(not because I'm a slack nappy changer either). Poor little chap., metanium sorted it out with 24 hrs.
DO NOT get it on any furniture of carpet though, it never comes off. God only knows whats in it!!

Sudocrem really stings when applied to sore skin so if you're using that beware.

another vote for metanium.

I always used metanium but I know some mums who have used egg white which has worked really well and quickly.