A Clear Fold? Middle Set flop action

[TheRempel]

PokerStars Pot-Limit Omaha High, $1.00 BB (7 handed) converter

Button ($165.15)
SB ($99.20)
Hero ($210)
UTG ($31.25)
MP1 ($146.55)
MP2 ($177.70)
CO ($116.05)

Preflop: Hero is BB with T[img]/images/graemlins/club.gif[/img], 9[img]/images/graemlins/heart.gif[/img], K[img]/images/graemlins/club.gif[/img], K[img]/images/graemlins/heart.gif[/img].
<font color="#666666">1 fold</font>, MP1 calls $1, MP2 calls $1, <font color="#CC3333">CO raises to $5.5</font>, <font color="#666666">2 folds</font>, Hero calls $4.50, MP1 calls $4.50, MP2 folds.

Flop: ($18) A[img]/images/graemlins/spade.gif[/img], K[img]/images/graemlins/spade.gif[/img], 6[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(3 players)</font>
Hero checks, MP1 checks, <font color="#CC3333">CO bets $17.1</font>, Hero ?

I probably should have bet out here for information. I have a fairly good read on the other player, though. He will often bet big on the flop with top pair and slow down on the turn.