What Causes Head Spinning And Dizziness Every Time After Waking Up?

everytime i get up from lying down i get a head spin and feel dizzy for about 10-20 seconds, i feel absolutely fine during the day, its just when i get up from lying. i am on the contraceptive pill and have been for almost 7 months and have never experienced this until now. i am 16; do you have any idea what it could be?