Hi Kirsten, I'm so sorry that Luna is feeling so terrible but it is encouraging that she has managed to keep down her food and hasn't thrown up for 24 hours. Prayers that she has turned the corner and will get better each day.
My cat Sneakers had a similar thing a few years ago. She vomitted everywhere for a day and I raced her into the vet. She was hypothermic at that point. They did every test under the sun (xrays, ultrasound, bloodwork, urinalysis) including exploratory surgery to look inside her bladder! They never did find out what it was but they put her on IV fluids and ran antibiotics through the iv lines for a few days and she just got better and better. We have no idea what happened and neither do the vets. That was 4 or 5 years ago and she is still in good health.
I'm praying that Luna is dealing with something similar that will pass. I wonder if she wouldn't do well with some antibiotics though if she doesn't get better in a few days.