Dull lower back pain just under ribs on left side. Only feels slightly better when laying down on my right side. Muscle spasms in the left leg also.

Back pain. It is hard to tell without adequate physical examination. This could be due many possibilities. A muscle spasm, strain muscle, kidney stone, pinched nerve due to disc problem. I suggest you check with your doctor for adequate medical history and physical examination. This will lead to an adequate diagnosis and treatment. Hope you feel better soon.