third trimester 😩😰

I am just entering the third trimester and my tiredness has come back with a vengeance in the first I couldn't make it through the day without have a nap mid afternoon. 2nd trimester I felt amazing no nausea I had a burst of energy. Now I'm finally entering the third trimester I'm exhausted all the time again. I can't stay awake past 8am and I can't make it through the day without a nap. Is this totally normal for the third tri? and if so why?