((((((((susierose)))))))))
I agree with BeautifullyMistaken -- it may be that you need to have your medication upped, or even changed. There are a wide range of meds and dosages out there and sometimes it takes a little trial and error to find one that works for you. It might be that you just aren't on enough of the citalopram -- 20mg isn't very much. I myself am currently on 225mg of venlafaxine (to give you an indication of how high the dosages sometimes have to go) and have an appointment to look into adjusting them again in January. Anyway, if you've been on the citalopram for a while, it might not hurt to get in touch with your doctor and see about an adjustment.
Also, I just wanted to add that I think it's really great that you're trying to keep your depression away from your son. I know that you don't want to worry, upset or burden him. But repressing your feelings too much is probably going to make them worse, in the long run. Have you discussed your illness with him at all? Kids are a lot more perceptive than we give them credit for, and he can probably sense that something is wrong. Explaining that you're ill might help take some of the pressure off of you to perform the "I'm fine" act 24/7, as well as to give him a reason for anything he may have noticed about your moods recently.
Hope this helps. Sending lots of hugs to help you through.