I fell over skating and smacked my chest on a kerb on a tuesday, on wednesday I was jumped on by a large rottweiler. On thursday, I had a huge coughing fit while carring a large box of books, something made a cracking noise, and I felt something shift, in my right side, just under my bra and breathing got uncomfortable. NHS direct just about ordered me to A&E, where they insisted on checking my heart (funny place for it, I thought) and informed me that I couldn't possibly have cracked a rib because I was too young and fit. My heart, to nobody's surprise, was fine. After that they basically lost interest and kicked me out.