GI symptoms/fever/dehydration repeatedly

Curious to know if anyone has had a similar experience. Rilee is 9 months old and repeatedly has bouts of lethargy, vomiting, diarrhea, refusal to consume water with ultimate dehydration. The last event, not even a week ago, her WBCs were most elevated 35K, fever of 103.5F, vomiting/diarrhea, refusal to take water, abdominal x-ray was negative as was the pancreatic lipase test. She was treated with a total of 4 boluses of subQ fluids, subQ antibiotics followed by oral antibiotics and a bland diet transitioning to her usual diet. She did well for 5 days then abruptly began with the same symptoms. I am perplexed as to the causative agent - she is an indoor dog who goes outside on a leash or in a fenced back yard with no flowering plants/bushes and no access to chemicals, etc. No other pets in the back yard. Not sure what the next diagnostic step would be aside from possibly an ultrasound and/or colonscopy. Not sure if this is a "usual" illness pattern in beagles. I would appreciate any directional guidance or personal experiences. Many thanks, Rilee's Mom.