Pain in mid left side of abdomen...?

I'm sure someone else will come along with a more definitive answer for you, but based on my experience, pain in that area has been caused by two things.

The first is just simply intestinal pain. If you google a diagram of the human abdomen, you'll see that parts of both the small and large intestine could be to blame. My pain was from active crohn's in my "descending colon".

The second culprit was when I was much younger and came down with a case of mononucleosis (mono). Mono can cause swelling of the spleen, which also happens to be located towards the middle of your left abdominal area.

I don't know if that helps, but I hope you feel better soon!