High data use while sleeping?

Recently I was charged for 16G of data within a 24 hour period, which apparently happened while I was asleep at a hotel. The carrier told me the charges occurred at 9pm, 2 am, and 4 am. I do not stream movies or music. I can't seem to get any answers from Apple or my cell phone carrier to figure out what happened. But I wonder if this has to to with iCloud. I've gotten messages on my phone daily saying that my iCloud is full and they want me to spend 10 a month for a bigger plan. I have looked at cellular usage and it just says "Safari" was the most used app. That doesn't tell me much... I wasn't on my phone at all during the night, I was asleep! Now I have to pay for data overage charges! I just don't understand how that sort of usage could occur when I wasn't doing anything. Yes, I close tabs and swipe away pages not in use. My average data use per month is about 3-4GB, so 16G in one day is ridiculous!