Matt Stafford's wife, Kelly Stafford, is apologizing for chucking a pretzel at a fan during the Rams vs. 49ers game Monday ... calling it all a "weak moment" and adding she was "embarrassed" by her actions.
TMZ Sports has learned ... the L.A. quarterback's significant other was in the stands during the Rams' "Monday Night Football" game in Santa Clara, when a fan behind her started trash talking.
According to one witness, Kelly snapped ... kicking off an altercation by hurling a soft pretzel at the man.