Yesterday morning i stretched my arms up and immediately felt extreme pain on left side mid back. strained muscle? still hurts often going into arm?

Probably not muscle: It is unlikely that a muscle strain is causing radiating arm pain. The pain description you have given is more consistent with a cervical spine (neck) issue. Fortunately, it is very acute and likely to improve spontaneously. You can take over-the-counter medications and apply heat or ice. If the symptoms persist, worsen or become disabling, see your physician for more testing and treatment.