Hi Shem,
It could be spleen. I am not one of the experts at this site, but my experience after about 15 months on Gleevec 400mg/day (since April 2017) is that on some days there is discomfort or pain. That seems to go with the territory. Fortunately it is minor for me, and when I mention it to my oncologist he says those are known side effects of the drug. Joint pain and stiffness, eye puffiness, slight nausea or upset stomach, leg/foot cramps, and pain or discomfort in the abdomen (left side, as you note) -- these are occasional symptoms that I have (some of them less common than others) and maybe a lot of us have. I have decided not to worry about it because so far nothing serious has developed, and I simply make sure I mention these things to my doctor on my quarterly visits to check blood counts and BCR ABL.
Drinking lots of water always seems to be the answer to everything here, so I do try very hard to do that!
Thanks for your question and best wishes to you.