Coccidia doesn't always show up on a fecal test, even multiple fecal tests. This happened to one of my cats and I remember it was very frustrating to keep seeing the blood streaked messy poop but all tests negative. When we used the cocciodiostats medication the problem cleared up after about a week. See below from the Mar Vista Animal Medical Center website on coccidia:

It should be noted that small numbers of coccidia can be hard to detect so just because a fecal sample tests negative, this does not mean that the pet is not infected. Sometimes several fecal tests are performed, especially in a young pet with a refractory diarrhea; parasites may not be evident until later in the course of the condition.

How is Coccidiosis Treated?

We do not have any medicine that will kill coccidia; only the patient’s immune system can do that. But we can give medicines called “coccidiostats” which can inhibit coccidial reproduction. Once the numbers stop expanding, it is easier for the patient’s immune system to “catch up” and wipe the infection out. This also means, though, that the time it takes to clear the infection depends on how many coccidia organisms there are to start with and how strong the patient’s immune system is. A typical treatment course lasts about a week or two but it is important to realize that the medication should be given until the diarrhea resolves plus an extra couple of days. Medication should be given for at least five days total. Sometimes courses as long as a month are needed.