Android Device Overheating

Hi there, I have made a game for mobile (currently just android) and I have noticed that the game has started to make my phone overheat after about 5 mins of playing, after 5 mins it usually does thermal throttling. I'm not sure if this is because of processes in the CPU or the GPU, I did have loads of Instantiates and destroys but I have gotten rid of them and used object pooling and it still overheats, I read that transparent objects use up a lot of GPU processing power and I have a couple transparent objects though I doubt that is whats causing it to overheat.

How do I go about identifying the cause of the overheat and how do I fix it?

P.S My game is 3d and just consists of cubes and an image background

Thanks