I had the same problem when I was your age. I grew very quickly and got to 6' 4 at 15. After passing out a couple of times I went to the doctor and was told it was a combination of a growth spurt and deficiency in vitamin c
(I think it was that anyway). It will stop after a while anyway but go to the doctor if you are worried. Also when you stand up do it slowly as the speed in which you are getting up can have the same effect.