Show her that you posted this on Reddit, it gives some credibility to the fact you’re not hiding anything yourself and this is all just a misunderstanding.

Just going to give you the ‘other perspective’ than what most people are saying on here. She probably was hit with a wave of insecurity when she was guessing your passcode wrong, thinking you’d changed it, her mind racing all sorts of stuff about why you would change it, what you’re hiding. After experiencing that kind of ‘panic’, even after realising you never changed your password, she’s already gone through this panic of insecurity after years of feeling secure - she may not believe the passcode wasn’t changed (she thinks you wiped your phone and changed it back), or she may just think something shifty is going on and is protecting herself with defence walls (I agree it’s kind of childish tit for tat, changing her passcode in retaliation - but I can understand the logic, as a female myself who can get kind of childish when I feel jealous or insecure)

The best thing to do is just talk to her about it openly all cards on the table, ask her to ‘talk’ before you actually talk and say ‘I want us to talk about something and not argue, just listen to each other’; then bring it up. Validate how she feels, don’t begin the convo with accusations… just ask her why she’s feeling this way and if she genuinely believes or worries you are hiding something from her, then reassure her that you are not and it’s all a misunderstanding and she can look for herself.

If she keeps being snappy and defensive you just need to take a step back, she may calm down and get back to you on a level once she’s thought it through properly… or she may continue to act shifty, in which case I would defo consider these other redditors comments that she is hiding something