I go through this a lot. It is pretty normal. If you are breastfeeding, hormones release into your body that makes you tired.
But, if you feel it is making you depressed or you feel weak as well, check with your doctor and have a blood test. You could be anemic, low thyroid or have a vitamin deficiency (I have vit d deficiency and was having this fatigue).
Also getting too much sleep can make you tired!
One thing that helped me, was taking my vitamins and setting a schedule and sticking to it. Take a shower and get dressed when you first wake up. It really helps! Excersize and healthy snacks throughout the day help. Celery sticks with peanut butter, cheese and crackers, rolled up turkey meat, apple slices and a handful of mixed nuts, toast with nutella, hard boiled egg. Eat protein with carbs... it helps with energy absorption so you don't crash and burn.