Sometimes My Legs Feel "ghosty", Like They Are Empty Inside.

Sometimes my legs feel "ghosty", like they are empty inside. I can't seem to put all my weight on them, ey don't feel stable, and buckle a lot. Today this is along with a coldness inside like I would have ifd I had just donated plasma and was getting the blood put back in. I feel this coldness when I'm dehydrated, but don't understand the leg weakness