TPGK facing river aggression

[Profish2285]

Normally river aggression is a real hand so this would be an easy fold, but I had no idea what to put villain on tbh. The fact that I checked the turn makes me look FOS I think, which makes this river a standard call, but Im unsure. Villain runs at 22/15/2.

Full Tilt Poker, $0.25/$0.50 NL Hold'em Cash Game, 6 Players
LeggoPoker Hand History Converter

SB: $43.60
Hero (BB): $77.30
UTG: $47.30
MP: $15.50
CO: $52.20
BTN: $57.10

Pre-Flop: A[img]/images/graemlins/club.gif[/img] J[img]/images/graemlins/diamond.gif[/img] dealt to Hero (BB)
2 folds, CO calls $0.50, 2 folds, <font color="red">Hero raises to $2.50</font>, CO calls $2

Flop: ($5.25) 3[img]/images/graemlins/club.gif[/img] K[img]/images/graemlins/diamond.gif[/img] 3[img]/images/graemlins/diamond.gif[/img] (2 Players)
<font color="red">Hero bets $3.50</font>, CO calls $3.50

Turn: ($12.25) 7[img]/images/graemlins/spade.gif[/img] (2 Players)
Hero checks, CO checks

River: ($12.25) A[img]/images/graemlins/spade.gif[/img] (2 Players)
<font color="red">Hero bets $9</font>, <font color="red">CO raises to $22</font>, Hero....