A few things.
Overtraining - doing too much or training every day
Insufficient protein, and I don't mean whey, real meat protein, you get additional vitamins from this, everything is in the correct balance.
Insufficient fat - If you reduce your fat intake is too low you will get protein malabsorption
Insufficient calories - You may not be eating enough
Supplements - Some supplements can mess with your mood, drop all of the training supplements and after a good washout see does it keep happening
Not replenishing stores - Try eating a banana, some carbs, some salt, and protein together after a workout. The banana will add some Tryptophan and sugar in for you.
A combination of the above