9 month old healthy boy cat started having diarrhea almost daily, please help :(

Hi! My cat, fox randomly had the worst diarrhea end of September. He went almost every hour. I took him to the vet and they asked if he ate anything, any new environment, new food, the answer is no everything is the same with him and he acts completely normal. They gave me probiotics and psyllium husk powder. He was on it for 2 weeks, and seemingly went back to normal….

Last week I noticed some diarrhea but thought maybe it was because I was gone for a few days and he was stressed. But I took him back to the vet regardless. They did full panel blood work and tested his stool. Everything came back as normal. They did prescribe him Amitriptyline for anxiety/he pees outside the litter box a lot and this medicine has been a long time coming for him, the diarrhea is a separate issue.

He has now been on Amitriptyline for 3 days, but I don’t see diarrhea as a side effect for that. Also, he started getting diarrhea again before he started on Amitriptyline. Now, he’s had diarrhea 3 times this morning. I don’t understand what is wrong??? He’s had the same food and environment for years now. This seemingly came out of now where. Should he go on a daily probiotic?? Is this is a sign of something bad?? Can someone please help, I can’t afford to keep going to the vet and have them say everything is fine. :(