Am I being unreasonable?

We have had the same vet for 17 years. He knew Dakota and Dutchess well, and the two dogs before them. He’s taken care of all of our cats, too.
At different times in those 15 years a I’ve had more money than others. Sometimes I could pay a bill in full they day of the appointment, other times I asked them to bill me. I always got the bill paid off within 30 days.
Here recently, in the last year or so, he’s started balking at billing me, and I don’t know why. I don’t know if he’s had some bad experiences with some patients so he’s dunning everyone, but it’s starting to piss me off.
Cato has a really bad ear infection and he’s perfectly miserable. I called to make an appointment, and knowing how they feel about billing, I gave them a heads up that I can’t pay them until Friday.
The receptionists (the vets mother) said she’d have to ask him and call me back.
Well, she called back. Said the best they could do is make an appointment for Friday.
WTF?
I said, “No. He can’t wait that long. He is in pain.”
She sounded really regretful and said she understood, but that’s what Johnson said.
Well, he just lost a customer.