Stomach and back pain mystery in 3yr old... ideas?

My poor 3.5 yr old isn't feeling well and I'm trying to figure out what this could be. I'm fairly sure she has TWO things going on, but maybe they are related?

Here's what's been going on:
- She had three potty accidents last week (she's fully potty trained and has never had accidents before this)
- on Friday she woke with a fever complaining of a tummy ache
- later that same day, she said she had a headache and sore throat
- on Sunday she started saying her back hurts - one side of her mid-back area, but now she says her entire mid-back area hurts, not just one side

We've been to the pedi and they determined that she has the coxsackie virus - hence the less-than-24-hour fever and the huge ulcers that have appeared in her throat and on her tongue.

We originally thought she might have a UTI or bladder infection but all tests have come back negative on that, except for a trace amount of blood in her urine. The pedi is now concerned about kidney stones, and we x-rayed today for that (no results yet). That just seems so unlikely to me.

But what else could be causing tummy ache and mid-back pain?? It doesn't seem to be having an effect on her appetite, although that's hard to determine because the painful sores in her throat make her not want to swallow. She's not constipated, or showing any other signs.

Does anyone have any ideas? Suggestions of things to look for?