Cold chills/shivers during WILD

Lately i've been having this chills/shivers that happen to me when trying to WILD during my relaxation after WBTB. After WBTB i will go to bed, lay down and start relaxing, as the relaxation deepens, in the middle of my back this waves of chills/shivers will start and then sometimes spread up to my head, down my legs or just back area. They feel as if you are slightly cold but the point is the room isnt cold. If i roll on my side and continue to sleep, i don't get them, only when i try to WILD.

This has been happening a lot to me for a while and i have no idea what these chills/shivers means or if they are some sensation associated with how close i am to enter SP, dream or something. I know theyare not vibrations for sure, since i had few experiences with vibrations and know how they feel and look like.

I will appreciate if someone has some knowledge about this to share so i can know what and why is this happening to me.

Hi,

It is speculational explanation that i've come up with:
''It might be caused by fact that when we sleep our body's temperature goes down?''