Could Clumpy Feces With Brownish Strings In An Infant Be Roundworms?

Hi, I think my 16 month old may have a type of round worm..? I thought I may have had an infestation while I was pregnant with him. When I did mine, the lab gave me 1 O&P stool test, just one (I've read there should be 3?) to check--I did the stool test and the lab said it was negative. I do see fibrous-looking things at the break of the feces of mine. But, just tonight for my son, he had a dryer and formed, but clumpy feces with a brownish stringy Q-tip length string that was independent from everything else. This is the first time I've seen anything and been concerned like this enough to search. He very frequently has loose stools and I'm always paranoid of such infestation not only in myself, but my son. Could this be me passing down a celiac disease that has been undiagnosed in me, to my son? or most likely worms or ?