It could be that a few quinoa grains from the prior week got stuck someplace in your small intestines. I believe that it takes between 6 to 8 hours for food to go from the mouth through the stomach and the small intestines into the large intestines. I maybe wrong on the exact time but it would difficult for the quinoa grain to travel the distance in only 4 hours unless you had some serious digestive problems.
I have observed that I can often see partially digested food consumed 8 hrs prior to the enema especially if it is a deep cleansing series enema. Under certain circumstance the enema fluid can get into the small intestines and thereby flush out partially digested food out into the large intestines and then be excreted. This does not worry me at all.