Hi kawkaw2,
If you have one of the older waterproof vinyl mattress protectors that don’t allow any airflow through the protector then they can certainly cause heat issues yes.
If this is the case then I would probably replace the protector. There is more about the different types of mattress protectors and the pros and cons of each of them in post #89 here.
There is also more about the many variables that can affect sleeping temperature in post #2 here.
Phoenix