Is there a good deal of swelling? That as well as purple/red splotches in the area are signs of a bone fracture. If only your muscle is what is hurting, then you might have torn a muscle or tendon. If you don't have either and since it's near your sternum, you may have damaged some cartilage, which is going to take a while to heal. You really should find someone who can identify the injury, but either way, if it's very swollen or gets swollen in a very short amount of time, you'll need to get to a hospital.
But anyway, I don't think I can tell what the injury actually is without actually being there.
