Is it normal to be so tired around 31 weeks? Most nights I fall asleep at 3 am and wake up at 12 pm.…

Is it normal to be so tired around 31 weeks? Most nights I fall asleep at 3 am and wake up at 12 pm. Could it be the heat? I don't go outside, but it's pretty warm in my house. Also doctor gave me prescription for low iron. Maybe that's it?