Is my iPhone 4 Battery Draining?

Well… I will get to my point straight way. I bought and iPhone 4 in October, 2013 for My birthday. I been taking my phone to school and even when i go hang out with my friends and the battery life at that time was Fantastic! It used to be like Usage with 5 hours and Standby with 18 hours. Well, at the start of this year, it decreased. I don't know why, My result for the start of the year was, Standby with 14-15 hours and Usage of 4-5 Hours! Well, I went to my Service provider with my Warranty to check what was going and they asked me my usage and stuff like that, and i answered them, they said it was normal. Then i updated my iphone to IOS 7.1, the latest update. When i updated it to the lastest IOS the battery started to get better and i was happy about it! But then… From Tuesday (8/April/2014) It started to drop badley! (Screenshot of the bad drain will be shown) I don't know what's going on? Answer these question below too please. I done ressettting and everything i could do!
-Is it problem wiith the Battery?
-Is it problem with the IOS?
-When is the next IOS gonna come out?
-Is their a way to solve this?
-Can you replace the battery from a Local phone repair?

Added (1). If it is the IOS 7 issue… Is there away for Downgrading it to IOS 6 without Deleting any messages or Contacts?

I think it is a iOS 7 issue. I could be wrong but thats what I believe. Thats why I'm just on iOS 6 still. XP

I think it's the battery. Buy a new one and see if it helps. Also, have you tried turning your brightness down? If not, try it.

My phone's brightness is low, I'm on iOS 7 (very latest version) and the percentage is still 100% for the past 3-4 hours.