Backdoor flush?

[iceman5]

$1/$2 NL 6 max

36 / 16 player (75 hands) opens to $7 UTG. I call with [Qs] on the button. I have $220 and he covers. The guy is running over the table but it appears its a rush of cards...not anything like him going crazy outplaying everyone or anything.

Pot $17. Flop [6h][3c]. He bets $8.50. Not sure what 1/2 pot means, but I suspect he has nothing. I call.

Pot $34. Turn He checks, I bet $21. He calls.
Pot $76. River . He checks. I doubt he has much. Maybe 77? I bet $21 again. Now he check raises to $139. UGH!

He either hit the backdoor flush or is check raising my weak ass bet. I think thats a pretty rare play at these stakes. Easy fold? Or do you make the big call?

[Triple B]

[Monkey]

[black_knight6]

most people don't CR here as a bluff...fold

[iceman5]