John Cena's Wife -- I Believe He Cheated On Me

John Cena's estranged wife is convinced ... the WWE superstar was bangin' around behind her back during the marriage ... and now she's on a mission to prove it in court.

TMZ broke the story ... John blindsided his high school sweetheart Liz Cena with divorce papers back in May after 3 years of marriage.

Now, Liz's attorney Raymond Rafool tells TMZ ... they've been getting tips Cena had been messing around during the marriage ... which, if true, could be grounds for Liz to challenge the prenup.

"We're definitely following up on every lead relative to his being unfaithful before AND after the separation," Rafool says.

So far, it's unclear if Liz's camp has found any real leads.