P.A.
My 21 month old is doing this....very funny. We use a little bit of sign language and so if I think that she means yes, I reiterate that yes in sign language is like you are knocking on a door. So if she does mean yes - she will very quickly start moving her hand as so....
I also try and stay away from the word NO and use Not right now or Yes, in a little bit or Yes, after the toys are picked up etc...
Good luck!!