My phone has been overheating and has started to develop a white glow

The battery reaches temperatures of up to 42C and has started to develop a white glow when it does overheat. It usually does it when I have left it alone for a while(up to an hour) and the glow extends to about 1cm from the edge of the display. It happens at least once a day but I don't know what the issue is. I have tried google and have found no questions regarding the problem with my screen that I am experiencing. It was bought around November 2021 from giffgaff's "new phones" section on their website, I don't know if this information helps so I thought I would include it. Any advice would be appreciated and thank you in advance.