Can someone please tell me

Can someone please tell me why I'm so exhausted! 4th pregnancy and last one! I'm 16 weeks and 3 days. I am also breastfeeding a 10 month old. This pregnancy has been completely different so far from my others. I don't understand why I'm just so exhausted from the time I wake up. Most days I'm lucky if I get the kitchen cleaned in the afternoon. My body feels so drained and I don't know what to do. Sleeping or napping is no option when I have a 10 month old (naps for 15 minutes twice a day if I'm lucky) a 4 year old (refuses nap before 3) that is always with me and my husband works 8 hours a day plus drive time. What can I do? I need to get stuff done and want to spend time with my babies not feeling like a zombie.