Woke up with dull, uncomfortable pain in mid-lower left back for past three days. Urinating fine haven't pulled anything. What could it be my kidneys?

Most likely . It's your back. If you had a kidney infection, you would have a fever and would feel quite sick. Kidney stones are usually extremely painful. Although you don't feel like you pulled something, i suspect you actually did. The best thing to do is for you to see a doctor and get checked out to make sure it is really just muscle pain. Good luck!