fish not eating...water levels all fine?

one of my zebra danios has stopped eating. it has happened before when the tank was at a unusually high nitrite level. i did water changes daily and by the fourth day, she was eating again. she was also pregnant at the time. she is once again pregnant (shes just huge) and stopped eating. the water levels are fine (ph is 7.2 there is no ammonia and no nitrites) and it has been five days. she is now swimming into the gravel frantically like she cant see or thinks she can swim down more. all the other fish are great community fish and dont bother her at all, but i dont think she will make it much longer. any idea what this is? she usually hangs out in one corner of the tank, but today, instead of swimming in the same stop, she was resting on the gravel barley moving until it saw me adn started swimming. i want to avoid anything like this happening to my other fish