I had another thread about this a few days ago. iOS 8 is using a LOT more data vs. iOS 7 with the same usage habits. Something is up here.
My LTE iPad has the free 200MB T-Mobile sim. I havent used it outside of wifi this data cycle and it couldn't make it two weeks before hitting the cap. I've had this iPad for almost a year and have never hit my cap. (I only have the LTE active for "find my iPad" In case of theft)
iPhone on AT&T Is having the same problem. Blowing through 100's of MB's a day where it used to be 10's of MB's a day. I had to struggle to stay under my 5GB plan where usually I use 2.5GB a month like clockwork. I never stream music or video.
My mother and uncle share a data plan and are both seeing much higher data usage, to the point they had up change plans to accommodate.
I upgraded to iPhone 6 a few days after my billing cycle rolled over and was not able to get my iPhone's cellular statistics lined up with my billing statistics perfectly to be able to try and narrow down what my issue is. My bill rolled over an hour ago and I reset the stats in cellular setting so I can keep an eye on it for a few days before 8.1 so I can see if I can make an educated comparison.
I've read numerous theories about what's going on. Some think it's while charging overnight, some think it's itunes accounts. I'm not sure, but something is definitely running wild on iOS 8.