Q: imessage turns it self off since up dating ios 8.1.3

Hello sinead90,

If I understand correctly your iMessage setting keeps turning off on its own after updating to iOS 8.1.3. I would start by signing out of iMessage on your phone:

Sign in with a different Apple ID on your iPhone, iPad, or iPod touch

 

1. Tap Settings > Messages.
2. Tap Send & Receive. 
3. Tap your Apple ID, then tap Sign Out.

Then quit the Settings app:

iOS: Force an app to close

Double-click the Home button.

Swipe left or right until you have located the app you wish to close.

Swipe the app up to close it.

When you have done that restart the device and test the issue again:

iOS: Turning off and on (restarting) and resetting

Thank you for using Apple Support Communities.

Take care,

Sterling