What Causes Abdominal Pain When Lying Down?

Hi, I'm 21 years old and have always been fairly thin. Yet I have this mass in my abdomen that has grown slowly over the years (I don't recall when I first noticed it, possibly 5 years ago) and it protrudes now to the point where I look slightly pregnant (I assure you, I'm not. Prior to beginning birth control my periods where extremely heavy, and sparse sometimes going three months without). Within the last year I've realized how hard it has become, and when I lay on my back it protrudes even more and I am able to push the entire mass from it's location (the center of my lower abdomen) to a space under my rib cage. It's painful to lie on my stomach, and I'm not quite sure what it is. Any possibilities?