Auto brightness is off on my iPhone yet the brightness keeps changing dramatically on its own very regularly

I just updated my iPhone from iOS 9 to iOS 11.2.1 and ever since the brightness keeps going up and down! Sometimes it'll go super bright then a second later very dull then back up again a few more seconds later and it's driving me insane! I checked and I've definitely not got auto brightness on so how can I fix it cos at times I can barely see my screen. I keep it at the highest brightness possible and when it changes itself it still stays looking like it's at the highest brightness if that makes sense

Because it's most likely on auto-bright.