What would you do? #61 - Facing flop 3bet

Full Tilt Poker $0.25/$0.50 Pot Limit Hold'em - 7 players
The Official DeucesCracked.com Hand History Converter

BTN: $13.40
Hero (SB): $55.15
BB: $59.35
UTG: $50.00
UTG+1: $30.00
MP: $63.90
CO: $50.45 - 12/8 / 5.2 @ 600+ hands

UTG+1 posts a big blind ($0.50)

Pre Flop: ($1.25) Hero is SB with Ts 9c
1 fold, UTG+1 checks, 1 fold, CO calls $0.50, BTN calls $0.50, Hero calls $0.25, BB checks

Flop: ($2.50) Th Ks 9h (5 players)
Hero checks, BB checks, UTG+1 checks, CO bets $2, BTN folds, Hero raises to $6, BB folds, UTG+1 folds, CO raises to $18, Hero requests TIME, Hero folds
CO is threatening stacks here. What does he have that I'm beating? AK? It's possible that he limps QJ, even KT may not be a stretch. I fully intended to check / raise this flop if given the opportunity. I don't want a Qx or Jx hand in here. Does CO limp 99 or TT with a posted UTG+1?

Simple fact is that CO has well represented that he has a hand with a lot of strength and is willing to get stacks in. Easy fold? What do you think?

Final Pot: $14.50
CO wins $13.80
(Rake: $0.70)