Loss of balance and leg control. This.

Okay so sometimes when I stand up, something strange happens to me. It is a pretty common thing to feel the blood rushing to your head when you stand but for me it's different. I get that blood rush feeling, then my vision goes blurry and I can only see either yellow or black. After that, I lose my balance and have to lean against something. I get the feeling that I'm going to faint. I usually recover within 20-30secs. That has been happening to me for nearly 5 years now and I've just learned to live with it. But recently, something else has happened. What I'm about to say has only happened twice. Okay so after I lost my balance a few months ago, my eyes closed and I started walking backwards in circles over and over until I enventually fell over and recovered. That freaked me out, but I just left it. Now around 2 days ago, I lost my balance and my legs started bending like I was jumping up and down without leaving the ground. Both of these times, I did not make my legs do this.