nutmeg
I may be way off base here, but I have had pressure under my left rib cage that has lasted several days. This happens from time to time and is a really strange feeling. I've looked at anatomy pics and seen that my pancreas is located in that area. My idea is that my pancreas is overworking or swelling (??) because I've eaten too many carbs and sweets. I used to be really bad about that. Perhaps the pain is just your organs/pancreas dumping some toxins?
As sallybb mentioned, it could be part of your colon but usually kidney pain is lower and felt in your back.
Hope you get some relief soon.