On and off quick dull (mild) pain below the right rib cage. I've had this before but now its going since last night. What could it be?

Gallbladder? Your liver and gallbladder are located under right rib cage. If the pain is after eating a high fat meal: fried foods, red meat, etc, you may have gallbladder pain. Try cutting back on all fatty foods. If you have worsening, or nausea and vomiting, you should see your friendly doctor. A physical exam ( laying on of the hands by your doctor), and possibly bloodwork and an ultrasound may be needed. .