This is more something you may have to answer. Are you still training at the time of the losses...is your diet adequate(at least maintenance), much more info is needed.
The only thing that really comes and goes that quickly is water weight fluctuations. Many times these "gains" are mistaken for muscle.
It is also my experience over the years that folks tend to think they have more muscle than they really do, and are shocked when dieting down. We all have partitioning factors genetically "uned" and no matter how much food you eat does in no way mean more muscle.