100NL - Folding a flush on the river, bad fold?

[Corpsebean]

UTG was Bummert, who is like 14/12 or something, don't have too much history with him. SB is like 25/15 and nothing too spectacular from what I remember.

Full Tilt Poker, $0.50/$1 NL Hold'em Cash Game, 6 Players
LegoPoker Hand History Converter

MP: $57
CO: $79.50
BTN: $123.30
SB: $99
Hero (BB): $100
UTG: $109.10

Pre-Flop: 5[img]/images/graemlins/diamond.gif[/img] 8[img]/images/graemlins/diamond.gif[/img] dealt to Hero (BB)
<font color="red">UTG raises to $4</font>, MP folds, CO calls $4, BTN folds, SB calls $3.50, Hero calls $3

Flop: ($16) 6[img]/images/graemlins/diamond.gif[/img] T[img]/images/graemlins/diamond.gif[/img] 5[img]/images/graemlins/heart.gif[/img] (4 Players)
SB checks, Hero checks, <font color="red">UTG bets $12</font>, CO folds, SB calls $12, Hero calls $12

Turn: ($52) 6[img]/images/graemlins/spade.gif[/img] (3 Players)
SB checks, Hero checks, UTG checks

River: ($52) A[img]/images/graemlins/diamond.gif[/img] (3 Players)
<font color="red">SB bets $41</font>, Hero folds


I thought SB was probably either calling with a FD himself or slowplaying a set, and UTG was still behind just incase he hit anything. River fold too weak?

Flop and turn play I decided to take it slow. I seem to be getting roasted in these kinds of pots when I play it fast on the flop in a multiway pot. If SB had folded I would have CR UTG on the flop, and got it in if possible.