Is she still saying it hurts?
Plenty of toddlers I know regress from potty training for a few weeks. (She's 3, right?) But the pain issue would concern me.
I don't know if you use homeopathy, but when Thomas was whining and uncomfortable while urinating, I used cantharsis (6c) and it stopped. I use it when I get a UTI too.
If the pain continues, I would see a specialist. That's just me though. You've had 4 kiddos so you probably aren't as paranoid as I am. :)
Hope it gets better soon.