Big all-in raise with medium PP on low paired board

[Bez]

Ultimate Bet No-Limit Hold'em, $.50 BB (10 handed) converter

saw flop|<font color="#C00000">saw showdown</font>

Hero ($38)
MP2 ($52.35)
MP3 ($45.10)
<font color="#C00000">CO ($49.25)</font>
Button ($118.55)
SB ($19.50)
BB ($24.85)
<font color="#C00000">UTG ($25.10)</font>
UTG+1 ($61.40)
UTG+2 ($23.50)

Preflop: Hero is MP1 with 9[img]/images/graemlins/heart.gif[/img], 9[img]/images/graemlins/club.gif[/img].
UTG+1 calls $0.50, UTG+2 folds, Hero calls $0.50, MP2 calls $0.50, MP3 folds, <font color="#CC3333">CO raises to $1</font>, Button folds, SB calls $0.75, BB folds, UTG+1 calls $0.50, Hero calls $0.50, MP2 calls $0.50.

Flop: ($5.50) 2[img]/images/graemlins/club.gif[/img], 5[img]/images/graemlins/spade.gif[/img], 5[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(6 players)</font>
SB checks, UTG+1 checks, Hero checks, <font color="#CC3333">MP2 bets $5.5</font>, CO calls $5.50, SB folds, UTG+1 folds, Hero raises all-in $31.50, MP2 folds, CO calls.

Any comments on the validity of this all-in check-raise? I didn't think MP2 would have bet so much with a 5, but looking back, this move didn't seem sensible with a caller. I just saw a decent sized pot which I could take down now or maybe get called by AK or a lower pocket. The more I look at this hand the worse it looks.