So one of the blinds has called a LP raise and now bets this flop. AK would have probably reraised preflop to end the hand then so as to not have to play out of position. The most likely hand i see playing this flop this way (unless your absolutely shure its a bluff, which you don't seem to be) is JQ KJ AJ. All these hands fit his betting pattern. If he was bluffing in the first place and caught your hand is weak to the point that almost anything he can be betting on the turn is beating you unless hs has something weird like A5 and thinks you're weak.