Q: Excessive Bandwidth use

I love that I can tether my iPhone and iPad to my Macbook Air. However, I was horrified to discover that in a matter of hours of entirely normal use - no movie downloads etc. - I had used up all of my allocated data on my iPad - nearly 5GB. Thinking this might be an error on the part of my ISP I tethered my iPhone to the Macbook Air. My horror increased when I've used up 11GB over a couple of days. Devices were turned on and off in that period. I received urgent messages from my mobile provider warning me of my excessive data usage. I have NOT done anything abnormal or unusual. Having done some research it appears that having icloud turned on may be the cause. Grateful for any feedback or experience anyone may have. I have not been travelling much lately so this has only happened after the installation of Sierra. Any ideas?