Is convex surface normal in cast iron skillet?
Pans should be completely flat, and producers are usually very good at getting it right. Even very cheaply made pans are usually sold properly flat. 

What you describe sounds like you warped your pan. When you heat a pan on a burner that is too small, it becomes convex when viewed from above, if you heat it on a burner that is too large, it becomes concave. I have also seen lots of claims that heat shock (too quick heating by placing on an already hot plate, plunging into cold water right after cooking) increases warping. With thinner materials, you usually can see the warp easily with the naked eye. 

I have never seen this happen in cast iron pans before (I assume here that it is really cast iron - some people call all iron pans "cast iron" although forged iron behaves differently and warps easily). Combined with you saying that you only notice it when heated, I wonder if it could be that your pan is not really warped, but only temporary developes a hump on the burner. Just try it on a fslightly large burner (which is supposed to bend the metal in the other direction) and see if it changes something. 

If the pan continues to be warped when used on a different size burner, it has to be hammered flat. I have never done it myself, but the Internet is full of suggestions. They say that it can get "very flat", I have no idea if that means enough that there is no pooling, or simply much flatter than at the beginning.