The condition of high cognitive load is comparable to what happens when you're in a job interview or a police station, where there are high stakes attached to your telling the truth.
Now, if someone else compliments you on the shirt and asks where you got it, you might have to come up with a false store to stay consistent with the lie about its cost (say, Old Navy, instead of Banana Republic).
That shirt is now the “Old Navy” shirt, and will have to be each time it comes up as a topic of conversation.
Once you start down the pathway of lying, you not only have to remember facts, but which facts you changed, and how.
Imagine if you told your partner that the shirt you just bought cost $25, when it really cost $50 (and you paid cash).
Native English speakers were in the lowest cognitive load condition.