I get nausea/crampy at the end of my colonic when mucus is starting to be drawn from the colon . Nausea from the enema might be similar. I get extra nausea when their is excess bile being produced during my colonic. Excess bile can cause diarrhea according to Wikipedia. Type in "bile" @ Wikipedia for the info. So my guess is the excess bile that is being stimulated from the coffee is causing the diarrhea+nausea. The nausea could be caused from mucus/biofilm/toxins being purged from the intestinal wall. According to my colon therapist it is very common for people to get nausea during their colonic because of the constant purging action of the colonic. Probably very similar to when you take a strong laxative. The stronger the laxative the more nausea your going to feel due to the intense purging.
If the feeling is uncomfortable try to adjust the dosage to your comfort level.