What did too much weight on chest do to cause discomfort/pain when breathing deeply or moving a certain way?

When I exhale/inhale deeply there is discomfort in my chest just between my , and when I move a certain way it can be just down right painful. The day before yesterday I had a good deal of weight (maybe 175 lbs) lying across my torso for about an hour. The discomfort started shortly thereafter and the pain at a certain movement, such as bending over, started today. I'm fairly certain the weight combined with the length of time was the cause of the problem but I just want to know what that combination did to cause the discomfort ( i.e. bruising on the sternum; though that seems a little extreme, it's the best I can come up with on my own.) I am 5'3" female, 127 lbs, 26 yrs, no history of heart troubles or asthma or anything like that. I am not really worried, just curious. Any plausible ideas?