Dull pain when i breathe located at left flank region. what would cause this?

Muscle pain: Most probably related to muscle strain or back sprain. Other causes would be kidney stones, urine tract infections or bowel problems.

Many things !: Hard to say for sure. But could consider things like lung issues, rib trauma, muscular problems, blood clot in the lung, collapsed lung, diaphragm problems, abdominal problems (liver, pancreas) less likely kidney infection/stones. This is probably not a disc problem, which generally would have more constant back pain and perhaps shooting pain down the legs.