Hi Anna,
No, 20mg is the minimum 'therapeutic dose' ie the lowest dose that will have a therapeutic effect. As one of the initial side effects of Citalopram can be increased anxiety, Doctors often start patients on 10mg if they are suffering from anxiety already to allow their bodies to get used to the medication before increasing to 20mg. This helps reduce the side effect.
Being put on 20mg is no too much - it should start working soon. I think the maximum dose is 60mg so you've got a way to go yet! I'm on 40mg and it's working wonders.
Good luck, and when you get anxious try to remind yourself that any side effects you may get are just your body responding to the medication and it will have a positive effect soon. I couldn't sleep at first either but I have no trouble with this at all now. Just hang on in there.
Take care,
Will