Something is still using cellular data. Go into the phone settings and look for data usage and see if it will show you what was using all the data. When you are using the phone at home keep an eye on the wifi icon to see if it goes off at all.
YouTube and Internet are eating my data.
What category of use is 'Internet'? Might this be some form of tethering? Most phones categorize use by application (Youtube), but the term Internet is very generic for a category consuming a lot of data.