It's not the protein or the fat. The whole story was the salt. When you ingest salt your body needs to use fluid to process it. If you're going to eat during a ride, choose much smaller snacks and eat them more often. You're lucky that your stomach didn't hurt like mad when you were riding post-lunch. That would have made me yarf.
I used to live near Katz's and I've been there a lot of times. The decor is terrible. The service is militant. But the food is unebelievable!