Half written txt messages deleting itself (sgs4 4.2.2)

[Android Question]

I am having a strange problem with my Galaxy S4.

Okay, so say I am writing a text message with the default app. If I lock the phone, (or let it time out to lock) the message has dissapeared when I unlock the phone again.

If I type a message then hit the back key, it is successfully saved as a draft.

This has been going on maybe 4 days now, seemingly out of the blue.

Any ideas?

Using: Samsung Galaxy S4, GT-I9505, running 4.2.2