panic attacks/fainting/citalopram

Hello wise people
I wonder if someone is able to give me some information or even a glimmer of light at the end of the tunnel? My 18 year old daughter (who has downs syndrome) had a nasty bout of shingles at the end of last summer during which she had a few fainting spells. She is over that know but it has left her with an absolute fear of fainting to the extent that she panics so much that she actually faints. This is happening up to 4 times a day and is very debilitating. Her other symptoms include dizziness, almost incessant shaking and occasional headaches. When she faints it's only for a matter of second but she is absolutely terrified. She then goes on to avoid things she associates with fainting. An example of this is that she fainted on the stairs and now will not go up steps, she fainted in the shower and is now reluctant to go in the shower, and it goes on. She walks extremely tentatively clinging on to me or whoever is near by. She is reluctant to go anywhere in case it happens again.
I have had her back and forth to the doctor. He initially thought of prescribing her something for immediate relief (valium I think), but her blood pressure was low so he wouldn't. He has now prescribed citalopram and she has been taking this for about 8 days (10mg). She has had some (relatively) good days, but we have also had some terrible days since she has been taking it and this morning her movements were really jerky (as opposed to just shaking) for a few moments when she first got up. Every time I think there may be an improvement there is a set back. So I wonder can anyone tell me if this is normal, is it likely to settle down? Is there any "over the counter" medication you can suggest to relax her out of this anxiety? Should we continue with the citalopram? She has also been referred to the psychological service and she has seen one of the nurses a couple of times for assessment, but no action yet. Any help gratefully received.