It could be that the thermal paste has started to "go bad". Meaning the paste may have dried up and become solid. If you have not made any changes to your PC in the past month then that is likely the cause.
You need to remove the heatsink and clean off all thermal paste residue on the heatsink, the GPU chip itself and anywhere else to see residue. A simple damp cloth can be used followed by a quickly wiping the heatsink and GPU chip with rubbing alcohol. Afterwards you need to apply a fresh thin layer of thermal paste. I generally recommend Arctic Silver 5 since it is good and relatively inexpensive.
Yea, and I believe that GPU won't use a thermal paste, but a THERMAL STICKER. Just make sure to not replace the thermal sticker with paste. But if it's paste, then use a new paste, if not, then a new sticker.