This bizarre occurrence?

This is hard to describe without sounding stupid, but I'll try. Once in a while (pretty infrequently), after I first wake up in the morning, when I'm walking around (not anything strenuous) for a moment, my vision is partly clouded by black splotch like things, and I feel very tired, sometimes dizzy, and almost out of breath. All of this subsides in a moment but my brother thought it might be serious. Although it does not occur often, it has been happening since I was about 9 maybe. I am a 15 year old female. I always attributed it to having just woken up, but I don't know. I usually get enough sleep as well. What could this be? Is this normal, and has anyone else experienced this before? Sorry about the description, its really quite hard to convey to other people. Thanks in advance!