yup.. same happened to me.
Check the battery usage see which app is the culprit.
I had one time encountered I airplane mode in cinema, came out drop 20%.
End up I check is YouTube app sucking 20%, not sure if it's a bug, but if you pause the video you are watching and press home, it will continue draining your battery very fast, so remember to close the app, or I think at least close the video you were watching and not leave it as "paused".
