Screen going black

I just recently got the iPhone 8. Whenever I’m in any calls (on any social media) my screen tends to go on and off with a black screen. This only usually happens when my phone is on 40% or under. Touching the screen and pressing the home button goes unresponsive whenever this happens, so to make it work I turn the phone off, wait a minute or so and turn it on, then I clear all apps and it is all fine. I do keep the “battery saver” on until it turns off when my battery is sufficiently charged. I’m just confused on why my screen goes black.