At least, I think it's the RAM. I'm not sure. I just got the phone yesterday, and I downloaded one of my favorite game I previously had on my other phone. It barely has MBs. All of a sudden, I seen something that looks exactly like the battery icon, except it's not that. Is that the RAM letting me know it's starting to go down? When I uninstalled it (the game), the icon disappeared. I then looked at the Task Manager, the RAM was almost maxed out.
Just close all other running apps. Maybe a lot of apps are running in the background which you are not aware of.