Rivered Flush vs Bluff or Nuts or Top Pair or I Don't Know

[toss]

Villain is a 4-tabler running at 26.7/21.7/6 over 60 hands. Normal game so far.

Full Tilt No-Limit Hold'em, $1 BB (5 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: 2+2 Forums)

BB ($41.10)
UTG ($74.15)
MP ($106.85)
Hero ($102.05)
SB ($43.15)

Preflop: Hero is Button with 8[img]/images/graemlins/spade.gif[/img], A[img]/images/graemlins/spade.gif[/img].
<font color="#666666">1 fold</font>, <font color="#CC3333">MP raises to $3.5</font>, <font color="#CC3333">Hero raises to $12</font>, <font color="#666666">2 folds</font>, MP calls $8.50.

Flop: ($25.50) A[img]/images/graemlins/club.gif[/img], 4[img]/images/graemlins/spade.gif[/img], 4[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">MP bets $13</font>, Hero calls $13.

Turn: ($51.50) 3[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
MP checks, Hero checks.

River: ($51.50) 6[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">MP bets $21</font>...

What do I do?