What Causes Dizziness While Laying Down?

I am 40, 5ft1 and 53 kilos. I have started feeling dizzy when I lie down - I notice this particularly when I am at exercise classes and lie down to do abdominal exercises. My head goes all swimmy and I have to shut my eyes to stop the swirling and nausea. Last month I was given anti-virals for a week to treat painful herpes in my mouth. In March I had a week of anti-biotics for pleurisy. I am on anti-depressants (20 mg citalopram daily) for anxiety and depression. Apart from all that I am active, a non-smoker, and generally (!) healthy!