strange birthmark looks like a tanner part of my skin?

I am 18 year old male and i have this strange birth mark type thing on my stomach. it is about the size of a baseball, but it is different from other birthmarks i have seen. it is fairly light in color and mostly looks like a tanner part of my skin. the outline is not very pronounced and gets faded toward the edges, i asked my doctor about it but they didn't really know what it was. Im not exactly sure if i was born with it or if maybe it developed later in my early childhood. Could it be a permenant bruise?