Flopped set - donk style?

[Fazz86]

First hand at this table. Been tilting so I decided to move up to this level and win my monies back. However, this hand I can see any K going this far. I cannot see him expecting me to hold a 5 after my raise. Obv he does wake up when the flush card comes and also he could easily have 88 but I cannot see expecting the worst all the time. The overbet is for some reason made me belive he had a holding like AK. Opinions are much appreciated.

Full Tilt Poker
No Limit Holdem Ring game
Blinds: $1/$2
5 players
Converter

Pre-flop: (5 players) Hero is CO with 5[img]/images/graemlins/heart.gif[/img] 4[img]/images/graemlins/heart.gif[/img]
UTG folds, <font color="#cc0000">Hero raises to $7.5</font>, Button folds, SB calls, BB folds.

Flop: K[img]/images/graemlins/diamond.gif[/img] 5[img]/images/graemlins/diamond.gif[/img] 5[img]/images/graemlins/club.gif[/img] ($17, 2 players)
SB checks, <font color="#cc0000">Hero bets $12</font>, SB calls.

Turn: 8[img]/images/graemlins/diamond.gif[/img] ($41, 2 players)
<font color="#cc0000">SB bets $41</font>, Hero calls.

River: 2[img]/images/graemlins/heart.gif[/img] ($123, 2 players)
<font color="#cc0000">SB bets $140</font>, <font color="#cc0000">Hero calls all-in $139.5</font>.
Uncalled bets: $0.5 returned to SB.

Results:
Final pot: $402