I've Been Feeling Very Tired The Last Three Weeks, I

I've been feeling very tired the last three weeks, I even went for a walk and once I came home all I wanted to do was sleep. This has never happened to me before. Also about 6 days ago I started feeling nauseous then it stopped after three days and now it's come back again. I also took a pregnancy test but it was negative. What do you think the problem is?