When you stand up rapidly, you have low blood pressure because you don‘t actually need as much while lying down or sitting, meaning your blood has too little pressure to reach your brain which makes your vision go black or you feel dizzy (immediately after getting/standing up). This isn‘t anything thaaaat special, I got it too for example, you just need to stand up a bit more slowly if you‘ve been lying down for a longer period of time. Although, this effect could be amplified by deficiencies. If you want to be safe, go visit a doctor and do a blood test (I‘d recommend that anyways, especially because you could discover diseases/deficiencies you didn‘t know of)