Villain is a multitabler who seems to have some kind of a clue how to play. I have him at 25/7 over 300 hands and haven't seen him check-raise a single time in that period. He seems a little passive and not very imaginative, but I think he's at least a winning player. UTG and Button are both really terrible, if that matters.

My first instinct was to fold, my 2 pair isn't a very strong 2 pair and I'm facing a check-raise by a solid player. But I ran my time bank down for awhile because I wondered what he could possibly have that he would play like this? I eventually WTF-called him because I just couldn't put him on a hand, I'm not sure whether or not I was correct to do so. Thoughts?

PokerStars No-Limit Hold'em, $1.00 BB (5 handed) FTR converter on zerodivide.cx

MP ($97)
Button ($103.65)
SB ($119.65)
Hero ($205.95)
UTG ($107.65)

Preflop: Hero is BB with 3, Q.
UTG calls $1, 1 fold, Button calls $1, SB completes, Hero checks.

Flop: ($4) 6, Q, 3 (4 players)
SB checks, Hero bets $3, UTG folds, Button folds, SB calls $3.

Turn: ($10) A (2 players)
SB checks, Hero bets $7, SB calls $7.

River: ($24) 5 (2 players)
SB checks, Hero bets $15, SB raises to $35, Hero calls ?????