Hi aero1,
If you have the 4-way cotton stretch cover, it is a cotton/rayon mixture and not too thick and quite breathable, so while you may be sleeping warmer using this (I’m not sure what you were using previously), it would not be common for this type of an encasement to be described as “warm sleeping” due to the breathable nature of the materials. But I have a few more hints for you that you may find helpful.
There are many factors that will influence your mattress microclimate, including the overall plushness of your product (softer mattresses tend to sleep warmer than firmer products). Also, take a look at the mattress pad or protector you are using, as that can greatly impact your overall comfort. Something using any sort of PVC/plastic/vinyl layer will reflect heat back, and even the “breathable” ones using polyurethane films can feel a bit too warm to some temperature sensitive individuals. There is more about the many variables that can affect the sleeping temperature of a mattress or sleeping system in post #2 here that can help you choose the types of materials and components that are most likely to keep you in a comfortable temperature range.
If you are very temperature sensitive, you may wish to look at some sort of a mattress encasement that is just a very thin cotton or polyester cover with a bit of elastin or similar flexible material in it. Those tend to have the least structure, be the most flexible, and most breathable.
Phoenix