Suddenly Dim Screen

[The Lip]

Yesterday, while I was outside, I opened an e-mail and about 5 seconds later the screen got extremely dim, like it was fading out when the screen times out (set at 2 minutes), except that it stayed that way for 30+ seconds. I tried turning into the sun, covering the sensor, and touching the screen, but nothing made the screen get brighter. Thinking that the recent software update may have altered my settings, I confirmed that my screen setting was not on "auto." I pushed the power button to turn off the screen, waited a few seconds and pushed it again. The screen came on to its standard brightness and stayed that way. I chalked it up to a fluke issue, rebooted my phone, and didn't have the problem again. Until today. Again, I was outside and tried to use my phone but the screen became so dim as to be unreadable. Any thoughts? I didn't have this problem prior to the VoLTE update.

Thanks.