Is it Android/LG G2 or ATT: Data overage?

Alright Android Army...Hope you can help me out with this issue. It's quite confusing.

Alright, I figured I'd post this on a few android forums I tend to frequent.

My mother and myself recently switched to ATT together back in November 2013.....She got the LG G2...I got the Lumia 1520 since i don't like android much.

Since switching to ATT she has gone over her data twice so far. We have the 1 GB plan. I remember at the store the dude was telling us to go for a higher plan but i felt it was overkill at the time.

We had the same 1 GB plan with Verizon and we were together...She had the Droid Razr Max HD i think it was(Can't keep up with android names) and i had the lumia 928. Never had any issue with the data going over.

Since switching to Att, this is now the 2nd month she has gone over and it is starting to become annoying to me because she is the one complaining.

I'm tempted to just go ahead and bump the plan up to the next HIGHEST PLAN they have since our data usage together with Att is about 1.2-1.5 GB of data.

But i am trying to figure out what exactly is causing the data overage? She has had the same plan with ATT that she had with Verizon.

I am smart enough to know to use Wifi for stuff. I don't use my data plan at all. At most i may use 100 MB-150 MB meaning the bulk of the data is being used by her.

She uses Facebook, plays a few games on her phone...I've also more or less told her never to use Youtube/Watch facebook videos till she gets home from work(It's hilarious to me how she goes over data when she should be working lol).

Anyway, these are the same thing she always did with Verizon. So what's the difference now?

I checked her Android Phone settings...Again I'm team windows phone and I dont know much about Android seeing as i've not owned one in over a year. But i know that the play store updates apps automatically and i thought maybe that could be why...except it was set to auto-update on Wifi only.

I also looked at what was using the most data and it says Android OS...Whatever that means? Even her most used apps are not the highest active data users. It's Android OS according to the phone.

I have no Clue what to do. At this point, I am about to either bump her up to the next plan OR i am going to tell her to move to T-mobile so she can use unlimited data and not have to worry about it. I feel bad for her having to constantly be worried "Am I using too much data?"

But something needs to give and i could use some the data usage coming from Android/LG G2 or something with Att?