If the skin isn't as flexible (it will be in due time) then exercise could cause it to stretch quickly from increased blood flow. This quick stretch could explain the pins and needles feeling because the skin is quickly being pulled on the micro-level.
Definition. During development, most animals form a gastrula. A gastrula is an embryo that has undergone gastrulation, or the process that produces different germ layers. Germ layers are the initial tissue layers of an embryo, and animals can have up to three. 1  Ectoderm is the outermost layer of cells.
1 The 3 germ layers-the ectoderm, the mesoderm, and the entoderm (endoderm): are in place at the end of gastrulation. The entoderm development is simpler than that of either mesoderm or ectoderm. 2  It is a monocellular layer lining the yolk sac until cephalocaudal flexion of the embryo takes place. 3  Flexion takes the embryo from a flat disk to its basic embryonic body form.