I was a cashier for a year until i started getting sharp pinching pains on my left shoulder. on & off for months now. i have no idea what it is.

Shoulder impingement: Sounds like what is called shoulder impingement. Typically caused by weakness of the 4 rotator cuff muscles-small muscles that help hold the upper arm into the shallow shoulder socket. Certain exercises can bring surprisingly rapid relief, but also need to be continued long term. Talk to your pcp, orthopedist or physical therapist (pt).