Back in the day, I used to be a nurse. There is what is called a vasovagal reaction that causes a drop in blood pressure. Needles, blood draws and similar things often trigger this.
Here is a Wiki link on it.
http://en.wikipedia.org/wiki/Vasovagal_response
If it only occurs when inserting the earring in the lobe but goes away afterward, I'd say there's a chance you're experiencing a mild form of this and not to worry. Just sit on your bed when putting them in. Your blood pressure is strongest to weakest from a lying, sitting, standing position. Of course if it worsens and happens at other times, go see your doc.
Happens to a lot of people.
Cheers,
Kalista