My brightness keeps dimming for several minutes frequently

So, I should say how this all started, I guess: This is a completely new phone. I’ve only had it for about maybe 2-4 weeks so far, and it had this problem since the very beginning. What happened was, before that, I broke my previous iPhone 6, which was when the screen popped out, and therefore, was going to be fixed, but upon this, the person who was working on the phone let me know that it broke, or something went wrong when that happened, so they gave me a completely new iPhone 6 to replace my old one.

I didn’t have the phone for about a week, since my mother didn’t want me to break it again, so she held onto it, and didn’t really ever leave it around anywhere. Then, when I finally get my phone, I begin noticing the issues. My mother had not noticed this, due to her already having 2 other phones, she didn’t really need my phone, but just made sure to keep it charged, and ready in case of an emergency. Anyways, nothing happened to it, but yet, every 1-30 minutes of me being on my phone, my phone decides to ‘shut down’ or at least, go and dim the brightness of the screen. I can’t see anything, and didn’t see anything until today, which I noticed it’s still on, just dimmed the brightness (I looked at it underneath the sun).

What I want to know, is there a way I can fix this in the settings, or will I have to get a new phone. I;m not much of a tech person that can fix things, but if there is a way that I can fix the phone on my own, please let me know. It would be better that I fix it on my own, rather than buying another phone again, and getting in trouble for it.

(if you need more information about how it does this/what functions when the screen is dimmed; everything works fine. I just can’t see anything in normal light or turn up the brightness when this happens. I have to wait a few minutes before using the phone again).