What Causes Pain In Rib Cage?

Hi I have been having a ache under my left rib cage for awhile now off and on but over the last couple of weeks its happening every day. It usually gets sore when I sit down in the evening and I can feel a kind of bubbling feeling in the upper left side of my tummy. The area that aches gets worse if I do more energetic walking and feels a bit like a stich.