Why am I so effing TIRED?

I don't know what it is with me lately, but whenever I go out with my kids for a few hours and do some walking (even at Target today) I feel tired and wiped out.  Yesterday I took Mermaid to a mall so she could hang out with two of her friends, and I did a *little* walking, but when I got home, I ended up sleeping for about an hour on the loveseat. I slept for three hours after getting back from 6 hours at the Rennaissance Fair last Friday! That is NOT normal for me. All I want to do right now is lie down.  

What gives?

I've been told I'm borderline anemic before, and a recent blood test confirmed it, but I am not usually this bloody tired after minimal exercise. My brain also seems to be on vacation, because I'm having a hard time remembering even every-day, simple words and names, sometimes in the middle of a sentence. WTF?