Fatigue

HOLA everyone! :D Hope that you're all doing great. SO here's my question and PLEASE send me some input about it... Sometimes when I do my workouts I get a lot of energy and other times I feel like I am about to pass out in the 200mts. Why is it so?? I try to concentrate on my breathing and still I get tired after each distance (more than 200mts). Does it have to do with nutrition or is it simply that one day or the other our bodies do not give that much energy? Hope to hear from you guys...:p