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 linemen 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.
Jayla said Larry's death left her "in complete shock," and now, she's not quite sure how she's going to pick up the pieces he left behind.
"I feel completely broken and don’t know where to go from here," Jayla wrote. "The pain of loosing you will stay with me forever.. You were my best friend and twin."
Jayla included several pictures of her and the former football player in her heartbreaking post -- including one that appeared to show her in Larry's arms minutes after she was born.
Jayla was one of many to mourn Larry's passing on Monday ... his former Cowboys teammate, Emmitt Smith, gave a tearful eulogy in an Instagram video. Troy Aikman sent his thoughts and prayers in an X post as well.
Larry -- who was just 52 years old -- is survived by his wife and his three children, Jayla, Loriana and Larry III.
RIP.