It's not your imagination. Your iPhone is using more data lately.
That's because in a recent update Apple quietly added a little feature that will use cellular signals rather than Wi-Fi if the Wi-Fi signal becomes weak. And it will do so even if the Wi-Fi signal is perfectly usable.
Most alarmingly: Your phone will indicate that it's pulling from Wi-Fi even when it's also drawing cell signals—so that trick of only using Wi-Fi for data on the road may not work and you'll have to pay for more data. The default is set so that it's always on, even if you have never chosen to do this.
Happily there's a quick fix. Go into "Settings" on your phone, tap "Cellular", scroll to the bottom and turn off "Wi-Fi Assist". Bingo: Problem solved.