Three weeks ago i pulled my hamstring yesterday i saw a bruise behind my knee its not where the pain of my pulled hamstring used to be is that normal?

Yes: A 'pull' refers to a tear in the muscle or its covering (fascia). This can result in a collection of blood to appear under the skin. As one is usually upright, the bruise tends to migrate diwnward.

Muscle snapped: Maybe a plantaris. If you are still limping after 3 weeks see your dr. Otherwise forget it.

Pulled hamstrung: Can be bleeding from small ruptured blood vessels as part of the pull.