Auto dimming (started yesterday)

[DonaRenee]

My phone has been great, up until yesterday.
I was playing a video inside the Facebook app and within seconds, the video dimmed to total darkness. I have to tap the screen every second or 2 to make the video reappear . This only happens in Facebook. I've tried it in YouTube and news websites and is fine. What can I do, is it Setting in the phone I've messed with or in the Facebook app itself

 

[Jakalope]

My S7 started doing this several days ago. Have been searching all over the settings and internet for answers but haven't found any. Does anyone know if this is an issue with Android 7, Nougat?