My Instagram account posted two photos by itself.

Hello,

I would like to know if it's ever happened to anyone that an Instagram photo posts itself? The photo is not part of my camera roll. If someone had logged into my account, I would have received an email. And I don’t think it’s hacking, it’s just a regular account with a few followers/following. I sent a message to Instagram and I'm waiting for their response.
When I archived the first photo, another one reappeared a few minutes later. I archived it again and immediately changed my password. And since then, nothing.
The photo was promoting a brand with a discount offer. As I said, I don’t have a large number of followers and my account is private.
I really don't understand, is this a bug or has someone actually accessed my account?

Thank you for reading.