I had red coloured stool with strong metallic smell 3 days ago. got a dre today, the doctor said there were no serious hemarrhoids. possible causes?

Other causes: Bleeding per rectum could be caused by either from the anal region (haemorrhoids, internal or external, fissure, etc.) or from above in the colon and is differentiated by the fact that with the former the blood is picked up when wiping the bottom or finding blood on top of stool and not mixed with stool (as it happens with bleeding from the colon)

Self-limited BRBPR: This may just be hemorrhoids from straining but bleeding from rectum (BRBPR) may also indicate anal fissures, proctitis (due to infection, inflammatory bowel disease, radiation therapy, stercoral ulcers, rectal prolapse, trauma, etc.) or a bleeding source further upstream. Consideration include age, risk factors, current medication (aspirin, nsaids, anticoagulants), known colitis, etc.