hi i had gone through your query and understand your concerns. i would come up with two possibilities to fit your condition .these include
1.the firs one include,ORTHOSTAIC HYPO TENSION; if you stands up from sitting or lying down, the body must work to adjust to that change in position. It is especially important for the body to push blood upward and supply the brain with oxygen
. If the body fails to do this adequately, blood pressure falls,and produce this condition
2.second possibility include.ANEMIA;check your Hb% levels for confirmation.
I hope this is helpful for you, thank you