Hey FresH,
More than likely you are having anxiety about quitting and it is affecting your stomach. I know my stomach was a mess when I first quit. I felt like I had a permanent lead ball in it. lol! I had that center pain too, mine was caused from my esophagas healing. Smoking causes a lot of irritation to the stomach and esophagus lining. The lightheadedness is usually because your body is changing drastically from the quitting and the nicotine is leaving. However, because you are going through some severe changes in your body, it might be a good idea to have yourself checked out. It is also a good idea to have your doctor involved with your quit anyway. Your Dr. will know what effects you may have from quitting.
Rather than panic, it will ease your mind to have your Dr. check you out. Let me know how you make out...and Congratulations on your two day quit!! Hugggs!