Sometimes there are monsters under the bed, sometimes there aren't. Is there a monster here? What should Hero do? Assume no reads.

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

Stack sizes:
Hero: $205.10
UTG+1: $440.50
CO: $225
Button: $364.55
SB: $188
BB: $138.20

Pre-flop: (6 players) Hero is UTG with A K
Hero raises to $8, 4 folds, BB raises to $14, Hero calls.

Flop: A 8 9 ($29, 2 players)
BB checks, Hero bets $20, BB raises to $84, Hero ???