Experienced players say that in this situation very often the opponent has a really strong hand such as over pair, two pairs, a set, a straight or another strong combination. Many players in the Internet prefer to play the flop slowly hoping to get some dividends on the next streets. The general rule for these situations would be:
If you have attacked during the pre-flop and the flop, but on the turn your opponent answered with a full counterattack, your opponent can beat at least a top pair with top kickers.
This rule in the first approximation can be taken as the main reference point.