Black dots appear in my vision when I stand up- what is wrong?

Circulation issue. You probably have some degree of lowered blood pressure which causes changes in the circulation to the eye leading to the appearance of dots and other phenomena in vision. Are you anemic or do you have some circulatory issue? Start with your general physician as this could be an early warning of a problem.
Low blood pressure. If you get up too fast, the blood pressure to your brain drops and your brain and sense organs do not work quite right. If it is too severe, you can faint. It is worse if you are dehydrated or have normally very low blood pressure. Some medications can have this kind of side effect too. Some medical problems with the heart can also cause this. If persists or worsens, see your doctor.