Originally Posted by froggy7
This may be hard for you to figure out, considering it was at the top of a stairs, but do you think that you stepped on him hard, or soft? If you stepped on the cat square-on, with all of your weight, I'd probably take him to the vet. If, on the other hand, it was more of a glancing blow, then I might be willing to check him out myself and just keep an eye on him in case things don't seem right.
Of course, the fly in the ointment is that you have only had the cat three days, so it might be hard to tell a change from normal in that case.