Recurrent soreness but potty trained

[FrankiesKnuckle]

DD is 3.5yrs. Successfully potty trained for months now, no accidents. Just given up pull ups for night time too.
At least once a week she gets very sore down below, from vulva to bottom, really red and angry, sometimes it can blister and she's visibly in pain (like tonight )
I just cannot fathom what causes it!
I tend to wipe her most of the time and I'm thorough but gentle.
As a baby/toddler in nappies she would get very sore from a dirty nappy very quickly. She had sore, blistered and broken skin more times than I'd like to remember. She was and still is bathed nightly in plain water.
Her diet is normal, she drinks water, milk and maybe a cup or 2 of very dilute squash a day. Bowels are regular and at least once a day.
I use metainium when it's bad but ideally I'd like it not to happen as it's so upsetting for us both.
Any ideas?