Just wanted to add; looking at this diagram; looks exactly where the spleen is, I dont want to jump to conclusions but could it be related to my spleen perhaps :| very scared what it could be

edit : http://www.Hughston.Com/hha/b_13_2_7a.Jpg
link doesn't work because I can't change the j to a lowercase, but if you could, the link will work. Thanks

Hiya, i'm soon to be seventeen and I have had a lump under my left rib cage for around 8 months now. It sounds similar to yours. Mine is a lump about the size of a bouncy ball that is directly under the left rib cage. Sometimes it sticks out so it is actually visible, this is usually when I stretch. It can cause some discomfort but it is not actually painful, it was only painful once while I was training ( I do alot of kickboxing training) I went to my doctor about 2 months ago, and he told me that it was cartilage and I have probably had it all my life. However he did tell me to monitor it and not irritate it. I'm not 100% sure that this his diagnosis was true but I feel a bit silly going back again!!! Get it checked out at your doctor, i'm sure you'd rather know that it was nothing than worry about it being something!! Hope I could help, let me know how you get on or feel free to ask any more questions! :)
xxx