my survival worlds keep switching to creative and/or deleting after a few days

I have created worlds in survival mode at least three separate times. I am able to play on/in them a few times, but then they switch to creative mode and some have disappeared altogether. I know of no way to save in pocket edition-there does not appear to be a save option. I have not counted how many times I can access a world before it switches and/or disappears. I don't know if there is something I am doing wrong being a new Minecraft user, or if this a problem in the game. I have a Nextbook 7 Android Tablet (I believe it to be the Premium SE version of the 7). Thank you!