Jayla Allen -- the daughter of Dallas Cowboys legend Larry Allen -- said in a social media post Monday night she's stunned over her dad's death ... writing in an emotional message, "I feel like this is a nightmare and I can’t [wake] up."
Larry -- a Pro Football Hall of Famer and one of the greatest interior offensive lineman the NFL's ever seen -- died on Sunday while on vacation in Mexico ... and his daughter seemed to indicate he had shown no signs of being in poor health prior to the tragedy.