I m having pain on my upper back on the left side under my last rib. it is dull and pulsates at times. it feels like a deep pain not a bad muscle pain?

Hard to say: There are multiple potential causes, you should consider being seen. Possible causes can incude an irritation of a nerve in the thoracic spine, an inflammatory condition involving the pancreas or the lining of the lung, and multiple other causes. A detailed exam and appropriate studies can usually sort out the cause. Thank you for the question.