I really need to vent.

Okay guys I have been with my fiancé for a little over 3 years now. I have full trust in my relationship until tonight where it truly has me questioning things. This passed weekend my fiancé had told said “ I’m so lucky to have a fiancé that doesn’t go through my phone and I can trust her” mind you we both had each other’s passcode to each other’s phone. No we never had to go through it but for comfort reasons and my passed with cheating we shared them with each other. Well tonight when I tried to send myself a picture that I had taken on his phone earlier I went to go send it and the passcode changed from a 6 digit code to a 4. And at this point I don’t know what to think or how to even feel. I feel sick to my stomach and don’t understand why he would just go change it like that. And then mention that this passed weekend randomly. idk what to think :(