My self harm scars hurt when I’m angry, upset, stressed or would have self harmed, why?

This could be a reactive sensation - your mind is 'used' to turning to that behavior when those emotions come up, so wether it does or doesn't end in self-harming, the sensation comes. Almost as if your body is preparing for it. I once read someone describe it as a 'ghost feeling'. If you notice in which emotions/situations it tends to happen more, that's a good step! It can also be a way to check-in and be 'self-aware' wether something has triggered you and if so, what. Not everyone will experience this, and even those that 'may' experience same sensation, may feel it on different ways. But you're not alone. This can also be an experience you have as you go through recovery and slowly 'disappear' or become less intense/frequent.

This may be because in your brain these emotions you are feeling are associated with self harms, therefore sending nerve signals to your body and simulating the same pain you would inflict on yourself when feeling that way.

The fact that your self harm scars hurt when you would have self harmed and when you are angry, upset, stressed, means that your body informs you that it doesn't want that you self harm anymore. It wants that you give up on self harm. Can you maybe consider giving up on self harming? Are there any organizations which can help you giving up on self harming? They can help alleviate your issues with self harming and also issues which led you to self harming. Can you maybe consider getting help in local support group or something similar for example?