I have had sever pain, like aching pain on my left side under my rib area. I went to the ER this…

I have had sever pain, like aching pain on my left side under my rib area. I went to the ER this past Saturday because it got so bad that I couldn't get out of the bed. I went in and all the doctor told me was that I might have pulled a muscle and he prescribed me some medicine for pain. But it still hurts. Do anybody know what this could be ??