Cat gets sick every time we go on a trip.

I have a 6 year old male neutered cat who is constantly ill either before or after we go on a yearly trip. This was never an issue until the last 2.5 years.

The first time it happened it happened after we got back from the trip. He wouldn't use the litter box or eat. We had to board him for a few days at the vet but he gradually improved. No issues for a year. Last year he had the same symptoms before we left for a trip. We almost didn't go but we got antibiotics from the vet and he was better before we left for the trip. No issues after the trip at all. We just took a trip in February. He was perfectly fine when we left for the trip and for a few days after.

He started having exclusive watery diarrhea and was seeking water from everywhere even though we have multiple sources of water. He was still eating although not his full amount, and he doesn't seem lethargic. He has dropped almost 2 lbs over the last 2 weeks. We took him to the vet for antibiotics, which only seem to slightly help this time. Yesterday we took him back in and and got his blood drawn for a blood test. Tomorrow we are taking him in for an ultrasound at the vet highly recommended. We don't have the results of the blood test yet, but hopefully will tomorrow.

I'm worried sick about what could be wrong and I'm preparing for the worst. This can't be a coincidence. This happens every time we go away, I can't understand why it started all of a sudden. We have always taken a yearly trip and he has had no issues in the past. I'm very concerned he hasn't bounced back like he has in the past. I'm just not sure what this could be. Does recommending an ultrasound mean that they think they found something and just need to verify?