PokerStars 10/20 Omaha/8 (9 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)

Preflop: Hero is CO with A 2 4 A
MP3 posts a blind of $10. UTG+1 posts a blind of $15.
1 fold, UTG+1 (poster) checks, 2 folds, MP3 (poster) checks, Hero raises, Button 3-bets, 2 folds, UTG+1 folds, MP3 folds, Hero caps, Button calls.

Flop: (12 SB) 5 , 9 5 (2 players)
Hero bets, Button raises, Hero calls.

Turn: (8 BB) K (2 players)
Hero checks, Button bets, Hero calls.

River: (10 BB) 6 (2 players)
Hero checks, Button bets, Hero calls.

Final Pot: 12 BB

i would 3bet the flop. as played i'd be pretty sure he has aces.

the stars 10/20 game plays very aggressively sometimes. there's not much of a reason to think you're behind here, unless you ran into the lucky ak25 or something like that. many players at that game will do this the same with a busted fd after they tried to get you off an a2xx hand.