Pain on my right side?

For about a month or two, I have had a pain in my right side that's underneath/behind my ribcage. I looked up a diagram and the pain seems to be where my liver and gallbladder are located. It is a kind of dull but constant pain. It's not a sharp pain, but it's daily now. Today I realized it hurt when I sat for a long period of time. I sat for 3 hours today because I had testing at school, and it began to hurt after about 2 1/2 hours. I ate lunch at about 11:30, and then had algebra cl at noon. After 15 minutes of sitting, it became worse. What could this pain be? I'm in 9th grade by the way, and haven't had any surgery or injuries. Please help if you have any idea what is going on. Thanks :)