I Had An Intestinal Parasite Last Year. Had Repeated Stool

I had an intestinal parasite last year. Had repeated stool sample taken and they all came back negative. My doctor finally believed me when a worm came out of me and I took it in to show him. I was prescribed a 3 day course of 600mg Biltricide (2x per day). I was fine after that for about 8 months. Then I started having the same symptoms. I went to my primary care and a GI doctor. Had 3 more stool samples, all negative. Now I passed another parasite, but this time smaller and flatter. Also a few string like worms in the stool.
GI doc said the amount of Biltricide I took last year was not enough to kill the worms and eggs. I am having bouts of constipation and diarrhea, lots of nausea, pain in the middle of my upper abdomen, and spasms. What is this?