Why is someone else's info appearing on my phone?

A couple days ago, my fiance's contacts, reminders, and passwords appeared on my phone. I saw some other, related posts, that said this could happen if I had synced my phone with his iCloud account. I've confirmed in my Settings that I'm still logged-in to my own iCloud account, though. Neither my fiance nor I have made any changes to our phone settings.

Our pictures, iMessages, and app downloads have not synced. So far, the only things I've noticed are that all of his contacts are now in my phone, all of his reminders are appearing in my Reminders app, and all of his passwords are stored in my keychain (which is concerning!). There could be other things that I just haven't noticed yet.

Any idea why this has happened and how I can restore my phone? Thanks!