If there is a sudden and abrupt universal change in response to commands and temperament, I would have her checked for pain. If her hips are bad, she will be reluctant to move and may respond grumpily due to pain. If not, never confront such a powerful dog head on, it is not wise. Forcing and roughness only teaches the dog that this is the way to communicate with you and yours. A leader should be firm, but not fully confrontational. Too many people misunderstand "dominance" which is a mistake that can be dangerous with a large grumpy dog. Practice the advise above with a matter of fact attitude, do not "square off" and do not be excessively rough.