There are so many variables here that could have been the cause, but I think its safe to say that there's very little chance that your plane was infected with bugs. The atmosphere on board an aircraft is hardly conducive for insects to survive, much less reproduce. I know most a/c that travel internationally have the interior treated with an insect repellant/pesticide, although I'm not sure whether WN does this.

It does seem much more likely that your bag could have bed bugs or other pests that came along.