Coccidia (spelling?), which is an intestinal parasite, definetly sounds like what that is to me. Awhile back, in fact one of the reasons I had found PT in the first place was because I myself had the almost exact same situation and needed information. The problem with my situation was, my sister and I had 3 different cats using one giant litter box and couldn't narrow down who it was. We finally discovered it was my sister's cat and she was diagnosed with Coccidia. The good news (if there ever could be good news with something like that...) about Coccidia is it's very easily treated with antibiotics.

Good luck, and hope your kitty gets well soon!