Unusually high data usage by Google Play Store
I recently had problems with my phone eating up an extra 4GB of my data plan in one month. Not fun. 

First try, Settings -> Apps -> All, and reset cache for each Google Play Services, Google Play Store, and Google Services Framework. Restart the phone, and watch.

If that doesn't do the trick, try resetting the cache AND data for each of the three. Again, restart.