What @PamPurrs said was spot on and @JoshFossgreen recommends always using a strap but I can say, from experience, a bad strap is worse than no strap at all.
Also, you can check these threads to see if they can help sort out what might be causing the pain and ways to relieve it.
Shoulder Pain Due to Weight of Guitar .
Harness Strap .
Hand/Arm issues? Problems? Solutions? Do tell - #28 by pukkita .
Problems with a shoulder .
I know that’s a lot. Unless it’s a matter of just getting used to it, like Pam said, it’s difficult to make recommendations without more information.
Good luck, @Gray and let us know how it goes.