Was she drunk last night? I wasn't sure, she seems to have rather a slurry voice most of the time. Most of them haven't had a look-in with the alcohol this season, with Bear & Co hogging and hiding it.
People either go two ways when drinking, maudling and glum or happy and exuberant. Marnie apparently sleeps with exes when she's drunk

Several of them in that house (mostly the 'squad' because they've had most of the drink) have hardly covered themselves in glory when drinking, so I think she's entitled to have one night of being tipsy and perhaps a bit 'off'.