Severe muscle pain left upper arm and runs all the way into pectoral muscle above left breast. how do i treat?

We need a better history & you need an exam. You assume it's muscle pain, why? Did you have a specific injury, a fall, or try to move a heavy object? If so, how long ago? If this were a simple muscle strain, a 79-year-old should be familiar with its natural course. Could it be a pectoralis rupture or something besides a muscle injury? Your question concerns me. I think you should see your doctor

Try an anti inflammatory. Good Luck