There is a condition called acalculous cholecystitis - which has mostly the same symptoms, but without stones. What did your ultrasound say about the thickness of the gallbladder walls (i.e., inflammation)?
If immune cells called eosinophils get involved, the results can be very destructive. Keep in mind 'eosinophilic disorders' if your stomach gets worse, and the esophagus (food tube) also - especially if food gets stuck on the way down.
Elevated BUN suggests kidneys are not working well.
Elevated white blood cells suggests infection/inflammation.
Ultrasound is very sensitive for detecting gallstones, but false negatives may occur if the gallbladder is partially contracted (which can happen if not fasting 4-6 hours prior to imaging). Visualization of the gallbladder may also be limited in patients who are obese and/or patients with abdominal wounds.