First to rule out the fairly obvious. If you are inactive and do a lot of moving around you could be triggering your heart to beat faster, which could be what you are feeling.
Now, down to what you are probably looking for, some problems. What this could be...diabetes
, or a congenital heart problem. Either way you want to get checked out by a dr. Perhaps record what you eat on days that you black out, see if there is a connection.
A congenital heart problem might include a leaky valve that when the blood spurts back the wrong direction (regurgitates) it could trigger your heart to beat much faster. Many people have this and for the most part it is a benign condition however if your episodes of blacking out are related it could be somewhat more serious.
Get yourself checked out and good luck!