The skid marks are from a small amount of bleeding. Blood doesn't come out with bleaches or ordinary laundry. It has to be removed prior to laundering. Spray them before washing with a product called Spray 'n Wash or equivalent.
The skid marks are rust. Blood is oxidized to rust particles by bleaches and washing. Rust is extremely difficult or impossible to remove.
You may have had some damage to the anal sphyncter which is not healing properly. It is impossible to tell without examination by a competent physician. When they check your prostate gland the idea is to not linger. They usually don't examine very well.
I sometimes have skid marks too. I had the flashlight routine also as a kid but with other tools. My doctor identified scar tissue a number of years ago.
Bleeding hemorrhoids can also cause the skid marks. Lots of people have the hemorrhoid problem. Hemorrhoids, if you have them, are not from your trauma but have another unrelated cause. There are dietary treatments for hemorrhoids if that is what you have.