Celexa does take a while to adjust and I also noticed the flatness and drowsiness, just as said above make sure you try and get a good sleeping pattern.
The effects should go away in a couple of weeks but if not see your doctor and they might want to put you on something else.
Hope this helps.
Personally, it sounds like the "adjusting" phase, Lithium works faster but the Celexa can take aroudn 4-6 weeks to feel it's affects. If this is your first go on psych meds, expect some drowsiness and flatness, though that could be just a phase of your depression. It's a guess, but I would give it at least a month. Make sure you set your alarm and get a healthy amount of sleep , as hard as it is, more then 8 hours of sleep can be bad for your. Make sure you set a routine up, even if it's simple things, that will help you get stable as well.