Some waterproof mattress protectors will make you hot and sweaty. It comes down to the type of waterproofing that is used, and how it is applied.
The waterproofing is in the form of a layer applied to the underside of the fabric. If that layer is a plastic or PVC it will make you sweat. To avoid this the waterproof layer needs to be breathable as well as waterproof. This sounds like an unlikely combination, but think of it like your own skin, it is both breathable and waterproof.
Search online for Allergon Breathable Waterproof Mattress Protectors. These use a polyurethane waterproof barrier. Polyurethane is breathable and waterproof. With this one the polyurethane is applied to the underside as a separate very fine film, this ensures that the backing is the same throughout.
Some mattress protectors apply the waterproofing in a liquid form that is pasted on to the underside. This method is less successful and leads to weak spots and thick spots and poor breathability, so always try to get one like the Allergon one mentioned above.