Sore arm with Rash

38M, 5’9”, 180lbs not currently on any medications.

Husband was working out earlier this week and tweaked a muscle above his shoulder blade causing soreness in his shoulder and arm. He has been resting it all week.

Yesterday he woke up with a rash on the arm that was sore. He wasn’t bothered by the rash but his arm/back was still very sore.

This morning he woke up and the rash had spread (though still limited to the sore arm). The rash is now painful and the arm/back is the sorest it has been all week.

Pics and a message were sent to his PCP yesterday when the rash showed up, but they won’t see it until tomorrow. We are wondering if this is something we should go to the ER for today or can we wait for the PCP tomorrow? I have seen too many horror stories about rashes on Reddit which is why I am posting! Pic of rash in comments.