[50NL] KK getting raised on river

[selector2006]

Button player is unknown.

PokerStars No-Limit Hold'em, $0.50 BB (7 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: 2+2 Forums)

MP1 ($42.25)
MP2 ($100.25)
CO ($61.70)
Button ($49.25)
SB ($36.90)
BB ($10)
Hero ($60.55)

Preflop: Hero is UTG with K[img]/images/graemlins/heart.gif[/img], K[img]/images/graemlins/spade.gif[/img].
<font color="#CC3333">Hero raises to $2</font>, <font color="#666666">3 folds</font>, Button calls $2, <font color="#666666">2 folds</font>.

Flop: ($4.75) 5[img]/images/graemlins/heart.gif[/img], 2[img]/images/graemlins/club.gif[/img], 6[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">Hero bets $3.5</font>, Button calls $3.50.

He calls but there's no flushdraw. Does he have an overpair? Or 56s? Or maybe flopped a set or a straight?

Turn: ($11.75) 6[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
Hero checks, Button checks.

He called me on the flop and I don't like it, so I check. Is this ok? Or should I bet and fold to a raise, or bet and when he calls check/fold river?

River: ($11.75) 4[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">Hero bets $7</font>, <font color="#CC3333">Button raises to $20</font>, Hero ??

Often I pick up the pot when I lead out on the river. But this time I get reraised. Looks too strong for me to call. Or is he just reraising because I was weak on the turn?