Actually am Having in the head a sensation of whirling, with a tendency to fall. I don't why this always happen when am trying to stand after sitting.?

Anemia. Young girls can become anemic if they have heavy bleeding during their periods. See your doctor for a blood test to make sure you are not anemic. Also dehydration can do this so drink plenty of water and avoid caffeine and alcohol (they are dehydrating). Take a multivitamin daily.