IPhone 6 battery suddenly dying faster?

Quick background:
I have the iPhone 6 (not plus), Sprint, 128GB, black front, iOS 8.3.
I'm very OCD about my battery percentage on my phone, I always have been. I'm in high school and my first hour is in a new building that had recently been built on campus and there's no signal ( I know bad signal kills battery). But, normally with my phone on airplane mode and grayscale, only listening to music for 40 min. And by the end of 1st period my phone would only be at 93% AT THE LOWEST.
As of last weekend however, my phone in 1st period (airplane, gray, NO music) without doing a single thing, goes down to 85%.
Dying quickly by the end of my 6hr school day.
Last night I went to bed at about 11pm with a battery of 41%, and I woke up at about 1am (2 hours later) and it was only at 80%. So, not only is it dying faster for no reason now, it's also not charging as fast as it used to. As well as going down in battery while in use on the charger.
Now, yes I have downloaded apps and whatnot, but I know it's not an app because it started over a weekend when I didn't do anything different. I have not changed my patterns of battery saving for 6 months in having this phone.

Batteries wear down with use, this causes them to charge slower and drain quicker. The only fix is to replace the battery… If this was a normal phone you could replace the battery very quickly for about $20 but since this is an apple product you will need to go to an apple store to get them to replace the battery (if you do it or even try to you will be sued)

It. An app, whether you think it is or not.

Uninstall some apps, mate.

My solution was to just let the phone die while in use, and it charged easily again! Doesn't seem to be dying faster than normal either.

Let's head to Settings -> General -> Usage -> Battery Usage. When you open Battery Usage, the first thing you'll see is the list of apps that have used the most battery life on your iPhone in the last 24 hours. For more search for dbxstore

Apple wants you to upgrade to new phone so they put out updates that drain battery's. They will say they did not do this, but phones with new battery's are doing this to. I took off all my apps, it still dyes fast. Hello to a non apple phone. Apple has screwed me the last time.