I am tired ALL the time.... what could be wrong?

I went to the Dr a few weeks ago, (for something different), and test results showed I'm healthy... it's just been the past week I have been crazily tired. I slept 10 hours last night, and I have been constantly tired all day. It's been like this for a week. So tired I can't even be bothered cooking food. My eyes have also started blurring at the strangest times.