Why did my phone use up 1.5gb of data in the middle of the night while connected to wifi?

Why did my phone use up 1.5gb of data in the middle of the night while connected to wifi? Iphone SE using 3g cellular with wifi assist turned off, backgroud app refresh turned off, cell data turned off for apple music app. OS is up to date. Only ever use apps that consume lots of data when I am on wifi. Reviewed my cell bills and for years have never come close to reaching my 2gb data limit until my last two bills came close and my current bill has put me over and my provider suspended my data because it went over by so much. Cell provider was useless saying only that it might have been an Apple Data leak? Any help with this would be GREATLY appreciated.