It sounds like you have found what we call 'The Bump'! 

The bump can be seen on the back of every single Inflatable board in the world. It is where the drop stitch material has to be cut in order to fit in the inflation valve. On some boards, the Bump is hidden under the fins, but is always there.

It is absolutely nothing to worry about, but if you are still concerned, then watch George explain it in a little more detail below.