. A cracked rib, especially during a sneeze in an awkward position is not only possible but more common than you'd guess. However, it also could be a bad muscle pull or even an inflamed bit of cartilage from the same basic cause, that sneeze.
The only concern with this, other than pain(which I realize is no fun in itself), is the restriction of deep breathing. Little is usually done for a fractured rib so long as it isn't free (as a blunt trauma to the chest can cause), but guarded breathing can sometimes cause problems such as pneumonia -- although if you are working through the pain you're probably in little danger of that. Still, it would be wise to do some occasional deep-breathing exercises against the pain, to make certain you don't lost capacity in the affected side.
Other than that, over-the-counter painmeds and whatever else helps increase your comfort level. If it doesn't get better withing a week of the initial injury you might want to bring it to the attention of a doctor, but it's unlikely anything will be done (although a doctor could prescribe something stronger for the pain).
I hope this is helpful.