Burning throbbing pain between shoulder blades, comes and goes, on muscle relaxer dosn't help, what's wrong?

Several problems! You may have strained your rhomboids. Another DX is subscapular bursitis, (yes your scapula which glide across the fascia of your back, have bursa!) scapulothoracic syndrome is another cause if you bend over while working with hands in front. The best physicans to see are a rheumatologist, physiatrist, or ortho-pod. These are remedial with exercise, nsaids, and perhaps a steroid injection.