What Causes Dizziness And Pain In The Back Of The Head?

I AM 65 AND HAVE BEEN GETTING DIZZY WHEN I GET UP FROM A CHAIR..ROOM IS SPINNING. IT HAPPENS A FEW TIMES A DAY. I WALK FOR 3 MILES EXCERCISE AND FEEL FINE ..SOMETIMES, I DO HAVE A SHARP PAIN IN THE BACK OF MY HEAD..LEFT SIDE.. I DO HAVE ARTHRITIS IN NECK AND BACK