Sounds like orthostatic hypotension, but best to check in with a health care person first. Have them check you over, bp taken and all, see what they say.

"Orthostatic hypotension means that when you are standing ("static") upright ("ortho"), your blood pressure ("tension") falls ("hypo"), and causes you to become dizzy or even pass out. There are several causes for this, including not taking enough fluids and salts, some medications (for example, the tricyclic antidepressants and the anti-hypertensives, of course), and some hormonal problems."

More info at:

http://casemed.case.edu/dept/autonomic/anstopic/oh.htm