What Causes A Big Rash Around The Chest?

Hi I just have been feeling different here lately and Have a Big rash going from my chest around to my back but I just let it be and haven't seen a doctor. It seems like every morning I wake up and my ribs will be aching and make me feel like if gotta go to the bathroom, And it also feels like my stomach is swelling right where my ribs are it is kind of scaring me now.