18 month old biting himself

For a couple weeks I’ve noticed my 18 month old will bite his arm in the evenings when he’s tired. He used to bite me when he was a little younger but he stopped because I would put him straight to bed if he did. He usually doesn’t bite very hard, enough to leave an indent but it would go away in a few minutes. But this morning he bit himself and left a mark. We were playing in his tent and having a good time so I don’t know why he did it. I brought it up to his doctor a couple weeks ago but she just shrugged it off. It’s upsetting because I don’t want to see him hurting himself but I don’t know why he’s doing it so I don’t know how to stop it.

P.s. he was born 11 weeks premature so he is a little bit behind developmentally but right where he would be if he was born on his due date.