Is my SO acting weird?

So I noticed that my SO has been really distant lately and changed their password on their phone and took my fingerprint off the scanner aswell and suddenly downloaded POF on their phone. When I tried to find out what was up they said I wouldn't be asking if I trusted them so obviously I don't and how is this going to work if I don't trust them, basically turning it around on me . They said that they only downloaded POF to see what was up cause they kept getting emails from them and that their profile is private and if they were on POF I'd know (but I've seen them scrolling through the site so it seems weird) and that they are distant because they aren't happy and are depressed (which i understand) but why would you change all the other stuff and keep POF on the phone? Am I going crazy? Is this weird behaviour ? Would anyone else be worried?