I think that Lauren was trying to be polite by saying that she was flaking out because she didn't feel well. If she had been being real, she would have said that she was too annoyed and frustrated by Josiah and needed to get out of there before she lost her frigging mind, but I think she was trying to be "good," "sweet," "polite" by blaming a headache, upset stomach, tiredness, etc, instead. It still came out in that talking head about building the nest, though, that she just really couldn't stand to continue the relationship building exercises with Josiah.
I think she's really disliking Josiah and really unhappy with this marriage. To miss the vow renewal in particular says a lot. Who knows what either one of them is like at home but right now they seem like the poster children for "marry in haste, repent in leisure."
Honestly, I think it might genuinely be too frustrating and painful for her to play at "happy couples" with a man who she can't stand.
What I also found interesting was how unexpectedly awful Joy and Austin's relationship seemed. I think maybe the miscarriage did wind up bringing them closer together, because in this episode...It seems like a very patriarchal marriage, in a way that I found kind of distressing, and it also seemed like there's likely a lot of unhappiness behind closed doors. Austin seems to be wound so tight and Joy seems so browbeaten.
Ben and Jessa also seemed to really not be getting along too well, but to me they still seemed very comfortable with each other in a way that the other couples aren't. Despite apparently being in a rough patch, they seem to me to have the most "normal" dynamic.
Even Jinger seems to be performing for Jeremy. If any of you watch The Marvelous Mrs. Maisel, they remind me of Midge and Joel's relationship in the very, very beginning of the show, before Joel left her. I guess instead of being "competing" comedians, they're competing influencers!