This is called Sleep Paralysis. Happens to usually everybody during WILD's and this is what usually keeps most people from accomplishing a WILD. Thus "dubbing" the WILD the hardest technique.
When you feel this, its normal. Go along for the ride and learn to enjoy it. It's like a roller coaster full of sheer exhilaration and unexpectedness. But when it's over, you are in a dream..already lucid.