What Causes Discomfort Feeling Under Left Rib Cage?

For the last 5-6 years I have had a feeling as if someone has taken a tennis ball and put it under my left rib cage. It is a squeezing feeling. I have been to doctors several times, and the emergency room, but when I go to the emergency room they say that my oxygen levels are 98-100% pump me up with Zofran/ morphine and send me home. I also have a taste of blood in my mouth in a.m. but cough up flem and there s no blood. It also seems worse when I eat.