Salam Ustaz, I hope you are well.
I have been going through issues on waswas and mental health for several years and in the process of healing. Do pray for me.
I will usually do istinjak several times on the toilet bowl and toilet floor for the second time. While performing istinjak on the floor, I had forgotten whether I have washed it clean after cleansing myself from discharges on the floor. I keep on feeling that I have discharges which forces me to stop listening to the whispers. At that time, I am not sure if the sprinkles of impurity touches me. So I just wore my undergarments and pray as I have spent long time in the toilet at that night. The next day, I saw dry stains on that undergarment. Should I just assume that it was pure when I pray? Since there is a possibility that the discharge can be secreted at a later timing, not when I am in the toilet or praying?
You must ignore the doubts as doubts are from Shaitan.
The ruling of impurity found after Salah is if when someone did Wudhu and Salah without checking and the size of the impurity is more than a dirham coin I.e. 50p coin or 5.94cm which is the circumference of the inner part of the palm, then after Salah it will be Fard to wash it and redo your Wudhu and Salah. If it is less than the size of a dirham as mentioned then Salah will be valid but Makruh and it is Mustahab to wash it.