Middle Back Pain

Okay...I have been suffering with this excruciating pain in my middle back for about 5 years now and nobody can tell me what is wrong! Every time I sit for an hour at the very least, the middle part of my back, only to the right side of my spine begins to hurt! That is the ONLY spot that hurts! Not the lower or upper back, just the middle right side of my spine! This is not from an accident either, I have no idea what caused this! The pain progresses until I lay down on my stomach or walk around for about 20-30 minutes. The pain is getting worse and worse and I do not know how to prevent it! I have tried everything! I have had 2 MRI'S, and X-ray, and physical therapy and not one person could tell me what is wrong with me. They found nothing! I am a full time student in my senior year of college and I sit in class most of the day and drive in the car a lot so... my back is hurting me all day everyday and I cannot take the pain anymore! Over-the-counter drugs do not prevent or take the pain away either! Please, if someone has a similar problem or might know what I should do, please help me! I am desperate, I cannot take the pain anymore!