I thought this was a pretty easy fold on the river, was I wrong? And I almost never fold sets.

Full Tilt No-Limit Hold'em, $0.10 BB (9 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)

Hero ($17.55)
UTG+1 ($9.85)
MP1 ($3.65)
MP2 ($19.35)
MP3 ($9.90)
CO ($10.10)
Button ($6.70)
SB ($3.80)
BB ($10.45)

Preflop: Hero is UTG with 9, 9.
Hero calls $0.10, 2 folds, MP2 calls $0.10, 2 folds, Button calls $0.10, SB completes, BB checks.

Flop: ($0.50) 9, J, A (5 players)
SB checks, BB checks, Hero checks, MP2 checks, Button bets $0.2, SB calls $0.20, BB folds, Hero calls $0.20, MP2 folds.

Turn: ($1.10) 2 (3 players)
SB checks, Hero checks, Button bets $0.4, SB raises to $1.2, Hero calls $1.20, Button calls $0.80.

River: ($4.70) K (3 players)
SB bets $2.1, Hero folds, Button raises to $4.2, SB calls $0.20 (All-In).

Final Pot: $11.20