I just started taking my anti-depressants again and..?

I recently stopped taking my anti-depressants because I thought they made me feel really weird. Well, i've bee in a really bad funk lately and today I came across my unexpired bottle of Celexa. I decided to take one this morning and see if it would help me get out of this depression. I feel a little antsy and and I can't seem to concentrate/focus. Is this a normal side effect?