I have a nagging pain under my left rib right in the middle. What could this be?

Ma be rib fracture. Did you heart yourself or no? If you did heart yourself that could fracture rib. If you did not that could be a lot of things, please check it out with your doctor. Good luck. Thank you.
Hard to say. Better check to be sure it's not related to your heart. Contact your pcp.
Chest pain. Any kind of chest discomfort may be cardiac, depending on your risk factors (smoking, high blood pressure, high cholesterol, diabetes, family history of heart attacks, age, weight). Give your doctor a call and/or go to an urgent care center, or emergency department. Most folks with chest pain are not having a heart attack, but you can have one with few or no symptoms. Please get checked!