If you are lying on the floor on or near Earth's surface on your back, I would take above you to mean above your ventral surface; that is, away from Earth's center, along a radius, out toward space.
Along the head-to-foot axis, or anteroposterior axis, "the polar opposite to the anterior end is the posterior end", your head being at the anterior end of your body, according to vertebrate directional terms in wikipedia.
The terms top and bottom are used ambiguously in your question, as top might refer to any of dorsal, ventral, or anterior, depending on whether you regard top as equal to spatial direction up, vs taking it as a body-relative term.