I started workouts / boxing type workouts about 3 months ago too and its happend to me a couple times aswell. My meals are pretty well balanced but dont know about you but i think its the fruit side which is lacking and i think its causing everyso often to burn out a bit. But also i think when youre relatively new it does happen. My friend who does alot of martial arts and rugby and christ loadsa sports lol he said it used to happen to him but he never gets fatigue anymore. Important thing is not to give up altogether so you had a bad day so what just get back into it next day!