Sometimes if i've been lying down or stitting down for a long time..

your equilibrium changes after standing up if you have been in a relaxed for a long time, makes you dizzy

That happens to me all the freaking time! So I guess it's normal

It seems as a matter of the so-called orthostatic blood pressure, a kind of idle running pressure that the heart ans major arteries maintain when in a fixed position. Normally, as soon as there's a significant change in body's position that requires of a bigger strength of the heart beat to be able to pull up the column of blood existing in the veins (which haven't pulse of their own as arteries have and so depend totally on the heart to move the blood inside them), the heart automatically and immediately will respond to the increased demand, but there are certain conditions that can alter this.
One is anemia, other, other is a ralentizing of the vegetative nervous systems, due to chronic tiredness and lack of proper sleep, or to a previous excess of stress, physiological or mental, or to a possible neurological organic problem, or a circulatory one.

Sounds like Orthostatic Hypotension, low blood pressure to the brain. Get a check up and ask your Doctor. It shouldn't be a big deal, but you shouldn't ignore it.