Left side back pain under lower ribs

[DaddyDad2]

I am a 50 year old male with type two diabetes. I noticed this pain as described in the title about 4-5 days ago.  I do not believe it's muscular because I cannot attribute it to any activity.  It was just suddenly "there." It is extremely tender to pressure and causes me to have pain with movement.  At rest it is a constant dull ache.  There is no visible bruising at the location either. I have an appointment Monday to have it checked out.  During that call to make the appointment, the nurse said I could go to the ER if I could not wait until Monday. Since the pain at rest is only a 3/10, I am trying to wait it out although certain movements or direct pressure can "send me through the roof."
Any ideas aside from muscular (which I have a hard time relating it to since I've done nothing out of the ordinary) that could cause this pain? Thank you.