Yea you must watch the donkey bet. Its not a Continuation bet because you were not the initial raiser. Playing a small pair out of position like that is really just set hunting. You will hit a set approximately 1 time in 7 so if you check fold when you miss and try and build the pot when you hit it can be profitable but only just.

Try to think of the hands the button might have played, its probably quite a large range, maybe 66+, A5+, K9+, QT+, suited connectros 56+, unsuited 78+, and their could well be quite a few others in the mix.
against this range 33 is about 47% pre flop and about 59% post flop.

A better play might have been a 3bet pre and then a CB. He would likely fold everything except a pair to this sequence.

p.s. The results are fab congats