Have you had your iron checked? What is your typical diet like?
I ask b/c I am normally anemic during pg. If I forget my iron supplements, for just 2 days, I am dead on my feet from the moment I wake up. It's brutal just motoring through the day.
If I keep an eye on my diet, and eat mostly whole, healthy foods, along with keeping up with my iron (and other vitamins) it's pretty manageable.
There are times when I am just done. My kids nap daily from 1-2pm until 4pm. Naps aren't optional. So if I am really dragging for a day, I will lay down and rest while they are napping, rather than cleaning (which is what I usually do).
Dehydration can also make you feel unusually tired.
Exhaustion is a normal symptom of pregnancy. But if you're exhausted more often than not, it may be worth looking into your diet, your habits, and your vitamin levels.