It's normal to get stomach-aches and poor digestion after eating any carbs after going RZC. I had that too when I was doing VLC for ages(I am surprised it happened so quickly for you lot, though). Basically, you need certain types of bacteria in order to properly digest carbs, and when you eat no carbs or hardly any carbs at all, those bacteria die off and you then get poor digestion and stomach-aches. Also, I think RZC makes the body change enzyme-production over time, or some such, making digestion more difficult.