That's very strange indeed, @Anonymous User. Is this something that you have been able to replicate, or did it happen one time?
I'd recommend checking this in the first instance, and also checking which fingerprints have been registered as well. You can then remove any fingerprints that may have been registered inadvertently, or remove all fingerprints and set them up again:
Please let me know how you get on.
It's not something I've seen reported elsewhere, so I want to help you figure this out, @Anonymous User. Do you happen to have Smart Lock enabled on your phone (just to see if this had some bearing)?:
https://www.samsung.com/uk/support/mobile-devices/what-is-smart-lock-and-how-do-i-use-it/
Additionally, what were the circumstances where your girlfriend managed to unlock your phone (e.g. were you using a particular app, was the display off, etc.)?
