Weird bump when I lean back.... What's that all about?

singingirl96

I am 18 1/2 weeks and my bump is well pronounced. I look further along than I am in my opinion.  I was a size 10 before baby, so I wasn't fat. 

When I lean back or try to stretch my back this weird, somewhat hard bump protrudes on the top of my belly,  I know that my uterus only is about as high as my belly button now, so what the heck is that bump?

Its not a major organ as those are hidden behind my rib cage.  

Is it my stomach muscles?  My top ab maybe?

Does anyone know what I'm talking about?  or am I just crazy?