Looking for an answer from a doctor... Should I seek medical attention?

Ok, i seem to have this condition where if i get up to fast, or even sometimes just getting up at all i feel light headed and my vision blackens out. (this lasts for roughly 5 secs.). My neighbor says it could be because of high blood pressure, but i don'y have high blood pressure. Also once or twice a month (full day) my knees become very weak and it is difficult to walk sometimes. Sometimes if it "hits" me while i'm running i will fall. By the way i'm only 14 (male). My uncle (blood) has problems with his knees like 2 or 3 times a week and i thought it might be hereditary. So should i go see a doctor? (please, only answers from certified doctors)