IPhone 4S keeps going into Do Not Disturb mode without me turning it on?

I will check my phone to see if I have any alerts and notice the moon in the top right corner indicating that Do Not Disturb mode is on (I will be calling it DNDM from now on). I don't remember turning it on, but the first few times it happened I just brushed it off as a possibility that I accidentally did. But it happens like once or twice a day now. I'm not even sure if the DNDM works when it mysteriously goes on because I will still receive texts, even if its unlocked (although I turned it to the setting to still block texts and calls while its unlocked). My iPhone 4S is like 2.5 years old so it may possibly be a technology thing, but I have all the current updates (7.1.2).

Anyone else have this problem or know how to fix it?
What could be causing it to put itself into DNDM?
Why wouldn't the DNDM work if it says its on, regardless whether I turned it on or not?

Its really not that big of a deal so I'm not going to take it into the Apple Story or anything, I was just wondering if this was a common problem. I can deal with it until the iPhone 6 comes out, thats for sure.

You might have it scheduled to turn on and not even know it. I turned it on once when I was in school to make sure my phone wouldn't ding but it became really inconvenient. Go to settings, and click DND thing and then see if the scheduled option is ticked