25NL AKo OTB vs unknown 3better

[Antinome]

Poker Stars - No Limit Hold'em Cash Game - $0.10/$0.25 Blinds - 5 Players - (LegoPoker HH Converter)

SB: $25.00
BB: $22.30
UTG: $9.55
CO: $25.70
<font color="black">Hero (BTN): $40.70</font>

<font color="black">Preflop:</font> Hero is dealt A[img]/images/graemlins/heart.gif[/img] K[img]/images/graemlins/diamond.gif[/img] (5 Players)
2 folds, <font color="red">Hero raises to $1.00</font>, <font color="red">SB raises to $4.00</font>, BB folds, Hero calls $3.00

<font color="black">Flop:</font> ($8.25) K[img]/images/graemlins/heart.gif[/img] 2[img]/images/graemlins/diamond.gif[/img] J[img]/images/graemlins/diamond.gif[/img] (2 Players)
SB checks, Hero checks

<font color="black">Turn:</font> ($8.25) 8[img]/images/graemlins/diamond.gif[/img] (2 Players)
<font color="red">SB bets $5.00</font>, Hero calls $5.00

<font color="black">River:</font> ($18.25) 3[img]/images/graemlins/heart.gif[/img] (2 Players)
SB checks, <font color="red">Hero bets $8.00</font>

Is this too slow? Should I assume an unknown at 25NL is 3betting me light assuming my button range is wide? I haven't stolen once at this table yet.