Alcohol and flexeril are both central nervous system suppressants and consuming alcohol while taking flexeril compounds the drowsiness, confused thinking, respiratory suppression of the muscle relaxant. Flexeril/alcohol combos are frequent causes of accidental overdose.
I would avoid the alcohol while taking the flexeril completely. If you already were consuming alcohol, I would skip the flexeril for at least 12 hours or more, depending upon how much alcohol you consumed.