Nisha had Giardia; sometimes it takes multiple fecal tests for it to "show up". The actual parasite has to be in the sample you take; which can be difficult sometimes.
With his weird sitting and walking, I would be concerned about a blockage
.
*on a side note; we had to feed Nisha a bland diet, the boiled chicken was getting so expensive! I bought Wellness Simple Solutions (duck). (rice and duck are pretty much the only ingredients). The vet okayed it as "bland" and she is doing great.