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Hi and welcome.
Since these set of symptoms developed recently, then this is not likely to be related to prostate
or urinary issue.
Still, it may be colitis, diverticulosis
or stomach issue, but considering weight loss
and stool diameter changes, I would recommend doing the colonoscopy
in the first place since some more serious issues may be present such as tumors of colon or rectum. Till then, please try to have high fiber food, a lot of fluids and probiotics.
Let me know if I can assist you further
Wish you good health