Brightness dims randomly even with Auto brightness turned off

I Have an iPhone 5s. the brightness of my screen changes randomly even with Auto brightness off. i always keep my phone on 100% brightness and sometimes The brightness gets dimmer. the brightness changes randomly. is it a serious hardware problem guys coz my phone is brand new. I went to Apple service centre and made a fool of myself coz I couldn't present my problem when asked. anyone else faced it before????