The natural thing for your doctor to do is to start with the most likely diagnosis and see what develops. It could be a transient ischemic attack, a pinched nerve, or some other things that are quite unlikely. Bell's Palsy is a diagnosis by exclusion, meaning it's just a name they use when they've ruled every other explanation out.

If it is a transient ischemic attack it would normally go away within a day at most. If it's BP, it can last longer. your age and the fact that you've had neither head pain nor other neurological symptoms suggests a stroke is very unlikely.

The doctor you saw may be right but he could also be wrong. He at least is being honest that he's not entirely sure. In that case, it's never a bad idea to go to another doctor, and see what he/she says.