Beautiful Tomcat with chronic digestive issue acting strangely irritable and aggressive.

My cat, Hank, has an chronic digestive issue that manifests as diarrhea and occasional vomiting. At times it has gotten worse and more regular, to an extent where he stops eating, in which case we take him to the vet, who prescribes Prednisolone. This seems to solve the issue temporarily. 

Recently, Hank has been acting strangely. While his digestive issues do not seem to be worse than normal, he has become very irritable and aggressive. He reacts negatively to sudden movements and hisses often at humans and his fellow feline alike. He also yelps and growls very loudly when agitated, despite there being no apparent reason. He clearly is feeling uncomfortable and I want to get him help, but am also on a budget and don't know if the vet will be able to shed any light on the situation.

If anyone has any insight or recommendations, please do let me know.

Thanks!

To me that sounds as though he's in pain. Something is hurting and yes, I would take him to the vet. Cats are very good at hiding pain and illness so if you're seeing it he's feeling it. I might be wrong but if in doubt it's always better to get a check done.