Crippling exhaustion at almost 16 weeks??

Adrienne
Hi all,
This will be my third child. I recall extreme exhaustion early in both pregnancies, I expected that. I'm almost 16 weeks though and I still feel painfully exhausted to the point where I can't function in the afternoon without a nap. A long nap. I don't even feel much better when I get up!! Anyone else still going through this? I am somewhat active, lots of walking (I live in NYC) and I try to eat decent... Any suggestions?!