Please help. In the episode Mystery spot, after Dean had dies, why did Sam have a hole in his chest with blood coming out of it?

He was shot! Yeah, it was too show how cold he became after Dean fully "died" & he just took the bullet out himself without any pain. He was looking for the trickster while killing other supernatural monsters without the aid of Dean. :(

Took me a second to realize what scene you were talking about.

It was six months later. Sam was wounded in battle. From what Bobby was saying, he had just taken out a vampire nest. I'm surprised the kid survived.

He got shot and he was stitching it.

The hole in Sam's chest was the reason Dean made a deal with the demon: Sam was supposed to die then, and he traded his eternal life for Sam's life.

That was dean, i believe.