“Doubt About Residual Istinjā’ Water vs. Discharge in Prayer”

After performing istinjā’ and completing wuḍū, I often feel during the prayer that some moisture or discharge has come out from the back passage. I am unsure whether this is actual discharge, or just some residual istinjā water that remained inside and came out afterwards. My concern is: If it is only leftover istinjā water, does it affect my wuḍū or prayer? If there is doubt whether it is water or actual discharge, am I required to repeat istinjā and wuḍū each time? What is the ruling for someone who repeatedly experiences this problem during prayer? I would appreciate clarification so I can pray with peace of mind and without constant doubt.