The visitors were mystified when referee Jon Moss pointed to the spot, having spotted a flag from his assistant, as David Silva swung a corner into the box.
It transpired Andreas Weimann had been penalised for handball as he leapt to reach the cross but the debate over what proved the game's major turning point raged for some time.
Lambert said: "That's never a handball. What the linesman saw there I do not know.
"You have got to be 100% sure to give that and he saw something I think 50,000 other people never saw."
Lambert added there was "no point" complaining to the referee.
He said: "We, as managers, are always hauled up - but that is a woeful decision.
"I haven't clue what he saw. It might not have changed the game but we certainly had a foothold in it."