Catelynn Lowell Talks Sia Invite

"Teen Mom" Catelynn Lowell got a personal invite to Sia's Detroit concert after the singer reached out to her ... she says because they have something dark in common.

We got Catelynn at LAX where she told us Sia was a fangirl of "Teen Mom" and singled her out on social media with the invite.

It's not the first time Sia's been motivated to help ... she gave an animal loving "Survivor" contestant $100k after he refused to kill his chicken.