As soon as I saw this, I was thinking gas, but you seem sure it's not. I tell you what though, why don't you just take a something like Gas X just to see if it makes a difference, it works wonders for my gas problems.
You can try the Gas X, but if you don't think it's gas you should definately go to the doctor. Sometimes when things are wrong with our bodies we can have referred pains. A problem with another organ may cause pain somewhere else is what I'm saying. So just go to a doctor. They can do x rays and run other tests to see what's causing your problems.
I know doctor visits can get pretty expensive, but it's worth it for something like this. When something hurts us and we don't know why, it causes us to stress, even when we don't realize it. If something is wrong with you stressing could making it worse, if it's nothing serious, stressing could still make you sick. So just go to the doctor. I hope this helps, and I hope you feel better.