Yesterday my upper left back felt really tight then I felt it crack and there was a loud squelching sound and now I'm in agony can anyone help please?

Emergency room. This doesn't sound like anything typical, especially in the absence of any trauma, so if you are in agony I would go into the er or at least call your primary physician and have them evaluate you.
See a doctor. If you are having severe back pain, numbness, weakness or loss of bowel or bladder control, you need to see a physician immediately.