Is your GF playing your account on your computer or another computer?

How do you shut your game down? Do you "log out" first?

When the game closes do you give it time to sync with steam cloud save? It can take some time depending on steam. For example having a laptop set to shut down when closed causes this issue a lot. Having a spotty connection with the ISP may also cause issues with cloud sync.. checking for ping and packet loss is a first step.

All that said, typically this is an issue with steam cloud saves. In the worst case you can just disable steam cloud save.

Have you ever tried to use any mods? Are you playing a solo world or on a dedicated server?