It could be bruising or irritation from falling on the floor or it could be an infection from the stress and injury of falling on the floor.
The main thing to do would be watch your water quality very closely (be sure to keep nitrates at least below 40 as a general rule and even moreso for injured fish, there should be no ammonia or nitrite). If he has a strong immune system he should be able to recover, but at a fairly old age after such a stressful event he may not, you can't know for sure.
There are no exact disease that fit that description so I could not recommend a medication and I personally think it would be best not to medicate unless you have something in mind (medications themselves tend to be stressful). There could be internal injuries as you mentioned and in that case I don't think there's anything you can do.
Last edited by MyBirdIsEvil; July 22nd, 2008 at 09:47 PM.