UPVC Door Keeps Locking From Inside After Closing & Others Can't Get In!?

Hi, I have a UPVC door with a multi lock which when you lift the handle from the inside, it locks the door from the outside so you can't get in.

In the past 2 weeks it has been causing a lot of problems where, when I come in the house & close the door behind me, it seems to lock & the person who comes in after me can't get it. I am not aiming the handle up when I close it behind me and it is only recently where it has been causing problems.

So far it has only happened after someone has come in the house, not when there is no-one inside.

Could it be the lock or door handle which is faulty? How easy is it to fix?