What Causes Numbness On Skin Over Rib Cage?

I am experiencing numbness on the skin that lies on my left rib cage. The area is about the size of a pop can bottom. I do not recall noticing it before today, the coloration is normal and there is no pain or itching, just a numbness (not tingle). What could cause this? I recently began tanning again but less often and at low time intervals and as of the last three days started heated yoga, which pushes my flexibility and I think could effect my nerves along my spin and those near my hips. Could either of these be a reason for this numbness? Should I keep an eye on the area for a while before looking into getting a doctors appointment?