Internal memory automatically decreases every minute, phone battery drains fast and phone heats up

[AC Question]

A couple of days ago, my phone suddenly notified that my internal storage was getting low, so I checked the contents of my internal memory as I am pretty sure that the apps that I installed in my phone were transferred to the SD card and that I do not use the internal memory that much. After observing the phone for at least 2 days now, whenever I will restart my phone, the free space will return to the normal amount of free space my internal memory should have. Then at least for every minute, the rate of my internal memory decreases at 10 mb and transfers either to the "apps" or "others" folder. On top of that, the battery drains so fast and the phone heats up. I'm not sure if its a bug, malware or virus but I would be truly grateful for anyone's help. Btw, I am using an HTC One M8 phone. Thank you!