Perhaps you have an ulcer already which is being aggitated by the lemon juice or cayenne. It could be a reaction to the cayenne pepper, especially if mixed too strong. It could also simply be hunger pains. In any case I don't think you need to worry about an ulcer after only 3 cups. That would have to be the fastest ulcer in history. As for the others here who have made some mention of problems they are having, you have to keep in mind, most of what you see here will be problems and people looking for advice. I doubt there would be enough room if everyone who was happily doing the Master-Cleanse posted just to say how good it was going. Most will simply continue without the need to ask questions. In my experience, and I have known many many MC'ers, this is a rare exception, not the rule. Most of the people I have known, inclusing myself, have had nothing but good experiences, even though of course difficult at times. But also remember every person is different. Everyone has different reaftions and tolerances. I would suggest drinking more water and keeping up with it myself, since it had only been 3 drinks. Though if it continues, even with more water and less pepper, perhaps you should see someone about it first. Could be something you never knew existed, or could just be a simple reaction.
Good luck in any case, I hope it works out for you.