pianoplayer - Ronnie mentioned what the problem most likely is, bare with a complete laymen and some _very_ loose analogies and I'll take a stab at trying to explain as I understand it.....
During our lives of slowly ingesting more and more chemicals and sterile substances labeled as 'food', our systems slowly build up layers of defenses against the onslaught. For a while, years on years even, this makes it easier for us to 'handle' the stuff (as our systems are shielded for a while) until it starts spilling over the defenses and causing health issues. Fasting and flushing help slough off some of the defensive buildup, a little bit at a time. One fast nor one Liver Flush won't clean out years of the stuff, it's a cumulative effect.
In the loosest laymen terms, for years your system has been wearing thicker and darker rose colored glasses about the stuff that goes through it, your fast and flush helped take the glasses off for a very little while (it's early, bad comparison, but maybe the idea comes across.) One round of cleaning years of junk won't take the glasses off for good though. Like any unpleasantry, the reality is a bit of a shock and mac&cheese & hotdogs Should be shocking (never, never, Never tour a hotdog making plant.) Given another day or two and some gentle after-fast foods like vegetable broth or juices and a gentle reintroduction of solid food like a few steamed veggies every hour or so and the mac&cheese etc wouldn't have caused such a reaction. Hopefully, after more cleanses and healthier food, you won't Want junk anymore. (I understand office food, though, and how _hungry_ one can get working the late shift - maybe next time you can take some fruit or a thermos of veggie broth with you.)
Every reputable or healthy fast warns about _slowly_ reintroducing solid foods and preferably good, healthy, easy to digest light solids first for a period immediately after the fast (24 to 72 hours seems to be the average) for this very reason (although they usually say so with much better medical descriptions! hehe) Some possible effects of adding too much or hard to digest foods too soon are bad cramps, elevated or wildly fluctuating blood Sugar levels, headaches, dizzyness, weakness (sure I'm missing some.) After a fast, your body has been without solid food for a while (hence the name) and needs time to readjust. Fasting isn't without it's warnings, but done slowly and carefully, fasting is a wonderful cleaning 'break' for the body.
Now maybe a real pro can come along and explain it much better. =)