I had a 5S.  I spoke with CS yesterday and we did some troubleshooting to make sure I have certain apps turned off. I do not stream videos or music. I am religious clearing out my apps as well.  I have maintained my same phone use for years as indicated by my data usage through November (it is all the same). 

I spent nearly 2 hours on the phone with CS this morning after the iphone 6 used nearly 3gb within a few hours.  I then decided to swap that phone with my old 5S that was on my other line that is rarely used.  The CS rep from this morning ran some reverse IP addresses and said it looked like I was bouncing off apple servers??  There is supposedly a ticket out there that tech support is going through the last 4 days of my cycle since it just started, last month with the overage to November my normal month.  I have used nearly 8gb in 4 days.  That is alot, dont you think?