Flop a set, in huge pot, but might be behind!

[WordWhiz]

Paradise Poker 2/4 Hold'em (9 handed) converter

Preflop: Hero is MP2 with 9[img]/images/graemlins/spade.gif[/img], 9[img]/images/graemlins/heart.gif[/img].
<font color="#CC3333">UTG raises</font>, <font color="#666666">2 folds</font>, <font color="#CC3333">Hero 3-bets</font>, <font color="#CC3333">MP3 caps</font>, <font color="#666666">2 folds</font>, SB calls, BB calls, UTG calls, Hero calls.

Flop: (20 SB) T[img]/images/graemlins/heart.gif[/img], J[img]/images/graemlins/heart.gif[/img], 9[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(5 players)</font>
<font color="#CC3333">SB bets</font>, BB calls, UTG calls, Hero calls, MP3 calls.

Turn: (12.50 BB) 3[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(5 players)</font>
<font color="#CC3333">SB bets</font>, <font color="#CC3333">BB (All-In) raises</font>, UTG calls, Hero calls, MP3 calls.

River: (17.25 BB) T[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(5 players)</font>
<font color="#CC3333">BB bets</font>, UTG folds, <font color="#CC3333">Hero raises</font>, MP3 folds, BB calls.

Final Pot: 21.25 BB

The reraise with 99 preflop is a tad frisky, but his was a somewhat loose aggressive table, so I didn't give UTG's raise much credit. However, when we go to the flop capped with 5 players, I figure I'm playing for set value.

Calling the flop seems a bit weak, but I considered this one of those circumstances where the turn would dramatically change my equity. If a [img]/images/graemlins/heart.gif[/img], 8, Q, K, or even an A comes, I could be looking at a made flush, straight, or higher set.
For all I know, I'm already looking at a higher set and drawing to one out.

So I call the flop, waiting for the turn to see if I should raise/ fold then. Instead, I chicken out and just call again. I'm scared because SB has done nothing but bet this whole time. But this was probably a mistake--I should raise and make AK or KK pay off my set.

River is fairly easy.

Thoughts?