IPhone locks itself.

Hello,

I have a problem, it's minor but it still puzzles me. Sometimes, my iPhone locks itself while I'm using it, whether I'm browsing the internet or on social media, and suddenly it locks and asks me to enter my passcode. It's not the automatic lock since I'm actively using it and then it just stops. My iPhone is up to date. It's not overheating because it isn't hot and it doesn't show any overheating screen.

Thank you for your help if you know why this is happening to me.