iPhone 6 Getting Hot And General Problems After Screen/Battery Replace

Let me give a bit of backstory. My IPhone 6 is not brand new and has been through quite a bit. I have dropped it in sandy water, dropped it quite a few times before that and, in short, I haveput this phone through !&&* and back.

Yesterday I brought my phone to a repair booth in the local mall and had them replace the battery, the screen, and clean the phone out. At first after this my phone worked fine but the day afterwards I am having a few problems. The very top of my screen underneath the call speaker feels very warm-hot depending on what is being displayed on it and the brightness. The heat also corresponds with more battery drain. Another problem I am having is some graphical glitches on my screen at certain battery levels/times. It looks like the phone is dieing but it is at 64% and I can turn it back on. I saw the screen flash black a couple times, flash a couple colors and glitches, and go dim. It was at this point I would restart my phone and then it might do it again. I am writing this at 38% and it hasn't done it since an hour ago at 64%.

I do plan to buy a kit and dissasemble and do a more thorough cleaning of my phone and all the connectors. I have read a bit on possible causes but would like so advice for my specific situation. Any help would be greatly appreciated.

EDIT: I did not see anymore glitches until the phone died. I am going to charge it all the way up and let it die again and see if I see anything. My phone also gets reeeally hot when I'm using my camera.

EDIT2: I just did a benchmark and everything on the phone runs perfectly well at the speeds they are supposed to. The phone did overheat during the benchmark but in a different location(right side of the phone) compared to when I use the camera(entire top of the phone overheats). I think the benchmark overheat is normal but the screen and camera overheats are not. When I get a fixit kit I will take a look inside my phone. Maybe I will even take it back to that mall and have the guy who replaced my screen and battery have a look at it because he is the cause I think.

EDIT3: The graphical glitches occur sometimes while the battery is draining or not at all. Locking the device and pressing he home button always seems to fix the screen glitches. The main thing I have been seeing is the backlight turning dark but I can tell the screen is still on. The battery lasts around 6-8 hours in normal which is perfectly fine. The overheating can be ignored but it is concerning. I don't use my phone for much more than browsing Instagram and skype. I don't usually play games on it. This is obviously some sort of a problem with a connection in the upper section of my phone that is causing the overheat. Thank you all so much for the help.