Why does my dog growl at me, but cuddle me?

Some dogs are more vocal. Rottweilers for example, growl at many things but it is not an aggressive growl. Think about all the dogs that growl when they get excited playing tug. It sounds like in your case it's just an excitatory reflex (at least while she's licking your face. Face licking is a submissive behavior so growling while doing that certainly isn't a sign of aggression). Take it as a compliment and don't go back to any trainer that tries to stop a dog from growling. If it WERE a warning growl, the last thing you want to do is stop the dog from warning.