"You have attempted to share a private, intimate image" - have to verify identity?

Facebook prevented me from logging in until I confirmed my identity. I haven't posted anything since 2016 and only use Facebook for messenger.

• I've checked my activity log and the last entry is from a month ago
• I've checked my logged in devices and it's only me
• I have 2 factor auth enabled so I don't think anyone has access to my account

How can I find out more information about this? Specifically why it was triggered and on what post?