If you feel a dull ache in back of your eyes after using the lamp then your eyes were exposed to UV for too long a time. I imagine the instructions warn against looking directly at the lamp.
As a comparison, UV light is emitted by arc welding. Looking at it with naked eye for a few seconds causes a headache sensation in back of the eyes. It goes away after a few hours
A similar 'eye ache' happens after exposure to bright sun for a long time. Strong sunlight may be reflected from a beach or snow. Sunglasses block UV to an extent.