I have pain in my left mid/lower back after eating certain foods (fats?) the pain is constant and dull and lasts for days. what could be the cause?

Not sure: It is unlikely related to the food. Do you sleep well or grind your teeth at night; if not sleeping well the muscles dont relax to allow blood flow in to heal a potential site of inflammation, this can cause a recurrent circuit that makes back pain easier to get. Mood can play a role, insomnia is a psychiatric "vital sign", use 2 aleve (naproxen) bid but can't think of a food assocaition, may get better answe.