diarrhea and parasites-anyone?

I often get diarrhea alternating with regularity and/or constipation. The last time I had chronic diarrhea, it lasted for more than one month straight! My doctors have attributed this to "IBS", but can't give me any explanation or way to fix it. Is it possible that I may have parasites living in my digestive tract? I was tested for giardia (which came back negative), but I don't think any other bugs were checked. Other symptoms I have are lots of rumbling and movement in my gut, uncomfortable and sometimes painful bloating, and foul-smelling flatulence. Non-digestive issues include anxiety/depression, nasal congestion, and lack of sex drive. When I use anti-parasitics, the diarrhea generally gets worse. What's going on here? Please help!