I think you have to consider outside factors i.e. stress, lack of sleep (which is a problem, i know),etc. If this is just a once in a while occurrence i wouldn't worry about it, but if it is occurring regularly then you're probably losing a lot of muscle along with the fat. The first thing I'd do, is up the protein. What type of diet are you following, because that is a pretty big drop in weights if it's been happening regularly.