That's most probably normal and should be explained in the manual which accompanies the instrument...
You start setting up the theremin on a microphone stand at about the height of your belly and make sure that you have a free space with a radius of 2m around the instrument to avoid capacitive side effects. Then, you position yourself in front of the theremin, somewhat shifted to the left side, so that the screw, which is at the left of the front panel points towards your belly. Then you step slightly back until the pitch antenna is at arm length from you. Set the pitch and volume knobs to their center position. Now, bring your right hand very close to the pitch antenna without touching it and move the hand on a straight line towards your shoulder. Pitch should start very high and then go down when you move your hand away from the antenna. It might be that the lowest pitch is reached before your hand is near the shoulder and it might even go up again. In that case, turn the pitch tuning knob somewhat counterclockwise and repeat the movement from the antenna you your shoulder until the so called "zero beat point" is exactly there and you don't have a so called "negative beat zone" (where the pitch goes up again). On the other side, if you can't find the zero beat between the antenna and your shoulder, you have to turn the pitch tuning knob slightly clockwise and repeat the movement until you find it near your shoulder.
It is perhaps a good idea that you take some theremin lessons from an experienced teacher, to better understand the instrument and to acquire a basic playing technique so that you will not find yourself blocked later with self-taught bad habits. Next occasion to have theremin lessons is here: http://academy.tfrenkel.com/pages/de/akademie-2014.php
