Like it on Facebook, +1 on Google, Tweet it or share this question on other bookmarking websites.
If it's severe you should see a doctor. I'm not a doctor, so don't take this as medical advice, just my experience. I saw you started a diet a few days ago. I sometimes get pain like that when eating poorly or changing diet. You might be detoxing. If the pain is up near your ribs it might be your liver. If down lower towards the back it might be kidneys. Make sure you get enough water, fiber, and maybe ease yourself into the diet instead of just jump all in.