My hamster had a big poo stuck in his bottom, I tried to clean it and it came away attached to a red string attached to the hamsters bottom. I am...

Several things you can do. Get a warm wash cloth and see if that will help stimulate he bowels as you wipe his rear end.  If he starts to pass his stool, and you still see a string, slowly pull the stool from his rear end until he no longer has any stuck. As long as he doesn't get distressed while doing this.  If you do this and nothing seems to be working, then I would seek medical treatment from your Vet.

This is something that's very common among hamsters especially if they've experienced any kind of stress. I would highly recommend getting him to a veterinarian who sees Exotics. Normally they will send home an oral antibiotic possibly Baytril. But this condition left untreated can cause severe dehydration and be fatal.