Black worm found in toilet...?

Alrighty, so after I had used the toiled, before flushing, I noticed a pure black, thick worm swimming through the toilet water. It was the size of a full grown caterpillar, and seemingly fuzzy. I nearly fished it out and examined it, but it squirmed around near my feces and COMPLETELY disgusted me. I couldn't bear the sight of it, and flushed the toilet... Umm, so what in the world was it? Any advice? Was it from my body, or did it swim up the line? Please help, I am very scared... I have always hated the thought of worms, let alone having one in my body...