Pain Just at Night?

Could be constipation. My daughter (at 3 years old) went through a similar issue. It ONY happened at bedtime. And it wasn't a stalling tactic. She would go to sleep (and she was a champion sleeper and no issues going to bed either) and then she would wake up an hour later in pain.

Essentially, impaction would press on her bladder, causing bladder spasms and that is some intense pain. Some nights it was just uncomfortable and some nights she was literally SCREAMING in pain... not nonstop---in spasms. It took a while for her to be correctly diagnosed, because she WAS pooping. She evidently just was not fully emptying, and would stop when the urge was gone, but before she was really done. Eventually, it caused impaction.
The spasms were intermittent, too, so it was really hard to get it figured out. No rhyme or reason to them. She'd spasm one night, then be fine for weeks, then have a week where she spasmed a couple of random nights, then be fine for weeks again. And it ONLY happened at night, usually after she was already asleep.

This is something to talk to your pediatrician about.

I'd have her checked out to rule out anything like a UTI or constipation - sometimes things are worse when you're inactive. A bladder is going to get fuller at night then during the day when she can pee regularly. And the bowel slows down at night too. She also doesn't have enough vocabulary or enough awareness of her internal organs to really describe it. If there is some constipation, try not to be forced into a medical solution over the long term - it's easy to get locked into some of that fiber stuff that's really junk. I work with a lot of kids who've suffered from constipation and UTIs, and there is a healthier, ongoing food-based regimen that's superior to medications. Good luck!

Since it's been two nights now, you have to get her in to visit the pediatrician/family practitioner. Most pain issues are worse at night. She might be having discomfort during the day but is able to ignore it, or she's not peeing during the day in order to try to avoid pain.

Get her to drink as much water as possible so that she pees frequently. Frequent urination helps flush germs. Also make sure that she's wiping herself from front to back, so that it's not discomfort from poop germs getting in the wrong area.

Sounds like pinworms. Time for trip to the doctor.

Pinworms come out at night, in the dark. I do not know how they tell it is night out since it's always dark down there.

It could be a UTI.
If she pees enough during the day she might not feel the irritation building up.
Does she use any bubble bath in her bath (or any other soaps that might be harsh)?
Bubble bath can be very irritating.
If she just had a bath and has the soap on her - that might account for the pain until she pees and that rinses some of the soap off sensitive areas.
It would not hurt to have this checked out by a doctor.
A urine test will tell you if a UTI is starting up - and at least it will be a relief to know it's nothing if it comes back clear.
Have her drink plenty of water and/or some watered down cranberry juice.
Go easy on any other juices or any sugary drinks.
Use plain bath water with very minimal hyper allergenic soap to wash her and her hair.
You can add 1 cup of plain yogurt to her bath water - it's very soothing to the skin.
I hope she feels better soon.