My family has a history of severe depression, and I always wonder if I'm just feeling down, or it's the beginning of something else.
Depression can be very normal if it's temporary, especially in response to major life events.  How long do you stay depressed?
Usually just for a few hours. A good nap sometimes helps get me through it.
Do you ever lose interest in activities that you used to enjoy?  
Sometimes, but it's more like I'll do one thing that I like, and then tire of it quickly and move onto something else.
It is very normal to experience problems concentrating, remembering things, or making decisions while depressed.
My memory is never a problem, but it's more like my mind is trying to tell me that I feel a certain way when I know that I should be.
I understand.  Sometimes that can make you feel ashamed or restless, too.
I don't think it's gotten that bad for me yet.  I just have to keep telling myself that Depression lies.
That certainly sounds like a good approach.