Glad to hear you're getting some good results, in comparison to where you were a month ago.
Stabilization on an antidepressant usually happens after three months. This is the amount of time the brain requires to regenerate parts of the hippocampus that were damaged due to previous psychological stress, or atrophy, which resulted in the brain's malfunctioning, with un-needed anxiety attacks. Since you've been on the Celexa (Citalopram) at five weeks, your brain is still adjusting, although you are already experiencing the positive side effects. Usually a doctor will prescribe you a benzodiazepine in the interim, when these anxious episodes like today occur, before the medication fully kicks in.
For me, I had used the benzodiazepine, Klonopin (Clonazepam), a few times over the course of my antidepressant three-month, induction period. Normally I used it at work, since I can't be anxious on the job. In fact, I still have a a prescription of benzodiazepines handy, in the event an unexpected anxiety rush takes over me. Since I only use a benzo every once in a while, I have a prescription for Ativan(Lorazepam), since it acts quicker than Klonopin (Clonazepam) to manage the anxiety. I've only needed the benzodiazpines once or twice recently, but I insist on having the prescription, since it provides me security that it's there, whenever I will need it in conjunction with my current antidepressant.
So yes, the anxiety at five weeks on an antidepressant is normal, where it indicates the Citalopram is still working in your system, however I recommend having a prescription of benzodiazepines, when dealing with these unexpected, raised anxiety episodes.