Did you use any tags in your post? And which tags if you did? Keep in mind that other users can report. If your bag appears in my timeline by misusing a tag I'm subscribed to it's an insta-report because man you can't imagine how many spammers there are pushing dropshipped goods from China.

And even if no one reported you if you don't have 2fa etc I'd say it's reasonable for FB to assume your account was sold, hacked or swapped.