As you probably know, reducing the frequency/dose of a
seizure med leads to an increased risk of having seizures
while your body adjusts to the change. However, awaking feeling groggy and stiff can be a result of many things, none necessarily related to epilepsy.
I suggest talking to the physician managing your condition medication. A sleep study may prove useful, or a 24-hour video EEG (in a hospital) or a 72-hour ambulatory EEG.