At your age, you could have a mitral valve prolapse
(not terribly uncommon in women). It would explain your symptoms, but so could some sort of abnormal rhythm that makes your heart beat faster than it should. When your heart is beating too fast, not enough blood gets to your brain, and you get dizzy.
The symptoms of either of those can be treated by medication.
It could be either one of those, or it could be that you simply have low blood pressure (also normal in women your age.), or be dehydrated. When you excersize, how much fluid do you drink?
It's better to know than not know, because then you can get help.
Good luck, God bless.