Was it really a cracked rib or just badly bruised?
Anyway the first thing that springs to mind is damaged intercostal muscle.
The intercostal muscles are like cushions between the ribs and are usually the cause of many, what doctors call "phantom pains" (no explanation) but they can be very hard to detect and don't show up too well on xrays you may need an MRI scan so if your next test doesn't produce anything positive you should suggest having an MRI scan which is so much more detailed than an xray.
If it is an intercostal then you will probably be given anti inflamatory drugs or creams, but for now you could try a hot water bottle held to the effected side for some relief of pain.
Hope this helps
anatomy & physiology diploma, First Aid Trainer, Indian Head Masseur