QK facing flop 3bet OOP [50NL]

[thing85]

Villain is unknown.

PokerStars No-Limit Hold'em, $0.50 BB (6 handed) internettexasholdem.com (http://www.internettexasholdem.com)

SB ($27.75)
Hero ($50)
UTG ($34.05)
MP ($49.50)
CO ($73.05)
Button ($24.25)

Preflop: Hero is BB with Q/images/graemlins/heart.gif, K/images/graemlins/diamond.gif.
<font color="#666666">2 folds</font>, <font color="#CC3333">CO raises to $1.5</font>, <font color="#666666">2 folds</font>, Hero calls $1.

Flop: ($3.25) 5/images/graemlins/club.gif, K/images/graemlins/spade.gif, 3/images/graemlins/club.gif <font color="#0000FF">(2 players)</font>
Hero checks, <font color="#CC3333">CO bets $3</font>, <font color="#CC3333">Hero raises to $7</font>, <font color="#CC3333">CO raises to $18</font>, Hero...?

Should I have folded PF? 3Bet?

What's my play on the flop with the failed C/R?

Thanks.

[Avocado]

pf is ok, but i lead this flop 100% of the time against an unkown. As played, raise more.. I make it $9 and fold to the reraise.