My Lumia 920 acting weird.

https://skydrive.live.com/redir?resid=175E990D28F3A0A!1549&authkey=!AGaDY6BSS49KJ1k


I woke up at 6:30 this morning, My phone was 100% charged till 7, then it started declining sharply, I didn't even do anything with it. It was just lying in my pocket in standby mode the whole time, when I took my phone outta my pocket at about 12 something, it was less then 50%, WTF. The same happened yesterday at school. Am I missing something or I just have a malfunctioning device. I use battery sense app to track my power usage and I've attached a graph that explains this.