SB is 18/8 over 50 hands, fairly aggressive. I don't think he is a regular. I haven't done anything unusual at this table. Hows the river bet size?

PokerStars No-Limit Hold'em, $2 BB (6 handed) Poker-Stars Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)

Hero ($222.80)
UTG ($212.20)
MP ($214.85)
CO ($224.60)
Button ($248.40)
SB ($233)

Preflop: Hero is BB with A, Q.
4 folds, SB raises to $8, Hero calls $6.

Flop: ($16) 5, T, 5 (2 players)
SB bets $10, Hero raises to $36, SB calls $26.

Turn: ($88) 8 (2 players)
SB checks, Hero checks.

River: ($88) 4 (2 players)
SB bets $55, Hero raises to $156