66 overpair facing strange donk bet (NL100)

[CalledDownLight]

So I play FR like 95% of the time and was a little lost here. This might be standard but I'm really not sure. I would usually fold here expecting to see 77-TT in FR. I'm readless. Whats my line?

Poker Stars, $0.50/$1 NL Hold'em Cash Game, 6 Players
LeggoPoker.com - Hand History Converter

BB: $53.75
UTG: $63.90
MP: $106.50
CO: $95.50
Hero (BTN): $115.85
SB: $86.90

Pre-Flop: 6[img]/images/graemlins/club.gif[/img] 6[img]/images/graemlins/diamond.gif[/img] dealt to Hero (BTN)
3 folds, <font color="red">Hero raises to $4</font>, SB folds, BB calls $3

Flop: ($8.50) 3[img]/images/graemlins/spade.gif[/img] 3[img]/images/graemlins/heart.gif[/img] 5[img]/images/graemlins/diamond.gif[/img] (2 Players)
<font color="red">BB bets $15</font>, Hero ???


edit: here is a hand we played earlier. He called me light, but I still am readless regarding his donk bet.

Poker Stars, $0.50/$1 NL Hold'em Cash Game, 5 Players
LeggoPoker.com - Hand History Converter

BTN: $130.05
SB: $98.50
BB: $94.10
Hero (UTG): $141.80
CO: $76.80

Pre-Flop: K[img]/images/graemlins/spade.gif[/img] A[img]/images/graemlins/diamond.gif[/img] dealt to Hero (UTG)
<font color="red">Hero raises to $4</font>, CO folds, BTN calls $4, SB calls $3.50, BB folds

Flop: ($13) 7[img]/images/graemlins/spade.gif[/img] 6[img]/images/graemlins/heart.gif[/img] K[img]/images/graemlins/heart.gif[/img] (3 Players)
SB checks, <font color="red">Hero bets $10</font>, BTN calls $10, SB folds

Turn: ($33) 9[img]/images/graemlins/club.gif[/img] (2 Players)
<font color="red">Hero bets $22</font>, BTN calls $22

River: ($77) K[img]/images/graemlins/club.gif[/img] (2 Players)
<font color="red">Hero bets $55</font>, BTN calls $55

Results: $187 Pot ($2 Rake)
BTN mucked 8[img]/images/graemlins/spade.gif[/img] 8[img]/images/graemlins/heart.gif[/img] and LOST (-$91 NET)
Hero showed K[img]/images/graemlins/spade.gif[/img] A[img]/images/graemlins/diamond.gif[/img] (three of a kind, Kings) and WON $185 (+$94 NET)