Could be anything from appendicitis to a stomach bug to a gallbladder problem. Pain is just the symptom of pretty much everything; it's how the body knows something is wrong.
Do you have a temperature? Pain or nausea or fullness when you eat? Any constipation recently? Is it closer to your belly button and below rather than above? Does it hurt when you go to the bathroom? Lay down and put your fingers tightly together, and press down below the ribs on the front right side, and let go quickly. Tell me if you have pain pushing in or letting go?
I'm not a doctor and therefore cannot legally diagnose you with anything. I can only give you a starting point to see your doctor about, just so you can tell him everything he needs to know (as in, more than just "my stomach hurts"). You really need to see one, just in case it is something serious. If your mother refuses to take you, then you will need someone else to take you instead. Bring your insurance card and have a friend or a friend's parent drive you if your father is too busy.
I'm not trying to lecture you; I know you most likely already know of all of this already, but I still have to say it. A majority of things that this could be requires some form of surgery or medication or other form of medical intervention to prevent death or other complications. You seem very nice; I don't want to see anything to happen to you. ;_;