Extreme fatigue

I got my Bfp last week!! I felt fine all week. Today I suddenly am so lethargic. I have to nap every hour and fall asleep while sitting up and while watching my LO. What are the chances I'm just coming down w something ... This doesn't feel like pregnancy to me.. I don't know what I'll do if this lasts for a majority of my pg!!! Any thoughts anyone experience anything similar