What Causes Dizziness While Lying Down Or While Tossing And Turning Despite Taking An Antibiotic?

I have been experiencing dizziness for approximately a little over two weeks. I have been to the doctor, suggested floanase twice daily and if no improvement in a week, they called in an antibiotic. I am on day 3 of the antibiotic and still am experiencing dizziness. it occurs when I am lying down and turn from one side to the other side and when I stand up from lying down. I just threw my head backwards while sitting up to see if anything happens and I was extremely dizzy. Today I added a Sudafed to my medicine intake hoping to resolve and other sinus issues. I only experience head ache and pressure when I lay down. I wonder if I am on the correct meds and what your thoughts might be as far as suggestions or concerns.