unfortunately since you just limped in with AJs, which IMO was a mistake, your opponent could have a wide range of hands. unless he likes bluffing away his money, i put him on a full house. he most likely flopped two pair or a set of 3s or 5s. other than that, if hes a donk hes on the flush draw. but im pretty sure that one of the two were slowplaying a monster flop.