Why do my eyes feel tired after I take out my contacts?

Typically dryness. There could be several causes for your symptoms but dry eye syndrome is yhe most likely.
Dry eyes. Contact lenses are essentially sponges that retain a certain level of moisture by pulling it from the surface of the eye. Dry eyes can result from long-term contact lens wear. Whenever the contacts are removed, the eye has no barrier protecting it from the air, which causes them to feel "tired".