You should do it again!
You shouldn't worry. Nothing bad happened as a result of your nap, and there is no evidence. You might however take this as a sort of sign to be more careful perhaps.
If you are called on it, say you were nodding but stood up so you wouldn't fall asleep, and that you will work on not letting it happen again, and ask the boss for tips for what you can do. Get the boss engaged on the future with you there.
Also, don't bring it up, let the boss bring it up if he knows anything about it. The 'client' might not have seen and might not mention it even if they did.
If it was for just a couple minutes, I wouldn't worry at all. In all likelihood nobody noticed, even if someone did it's doubtful that it will get back to your boss, and even if it does it's almost unfathomable that you might get fired over such a small first offense. I'd just write it off as a fluke (and make certain it never, ever happens again).
This is funny, but also pretty good advice if you don't decide to tell your boss yourself.
Just fall asleep with a pencil in your hand so it looks like you are writing.