I would 4th getting the d-dimer test. The chance that you have a blood clot is low but the consequences of missing it are BIG.

Having said that it sounds like a blockage of normal rib motion that could have been cuased by your back pack. Once you've ruled out the nasty stuff, go see a DO or PT that is trained in manual therapy, they should be able to help. Right now, you could try placing a tennis ball at the wall and leaning against it with your back but at the same level where the chest pain is or slightly higher. Roll the ball around the area with your back and see if it bings on your symptoms or hits some trigger points. If it does, it's even more unlikely that you have a clot and you might be able to ease you symptoms some with this type of self massage.