I had that too - it wasn't noticeable until after I'd given birth, and it was pretty hard, almost like a belt buckle sized rectangle.

I actually started a thread about it! Maybe March 2012 ... ?

Anyway, someone said what it was called (I don't remember, sorry) and it was basically liquid, which slowly but surely goes away. It took a few months though, so if it's that, don't stress, but don't hold your breath either - it takes a while.