I have no clue what it was but, sad to say, you probably have parasites anyway since evidently everyone does - including those doctors who might well pooh pooh the whole idea.
See link below for cayenne pepper challenge test to see if you have tapeworms, for example.
says everyone she ever lab tested had Tapeworms
& everyone had other kinds of parasites as well.
What helped me in case any of it can help you:
Btw most regular lab tests seem worthless for catching parasites. Parasites by no means exist only in the bowels & those that do evidently can easily avoid the stools - plus we know they can sometimes burrow deep into the intestinal tissue where they ain't budgin'.
Many people on CZ have reported getting negatives on stool testing, then seeing tons of them in the toilet after using a good paracide. I have seen at least 3 people report that they put live, wiggling, worms on top of stool samples & then were told they had no parasites!
If you see those white rice, or any other parasites, it's better to rejoice than get freaked out. You have found out some of your culprits! Most people will go a lifetime never having a clue.