Teeth move throughout your lifetime. However, the movement usually causes crowding. You are concerned about spaces, which is not typical.
If you had braces a number of years ago, your teeth may be moving as a result of that treatment. Or, you may have developed periodontal disease (gum disease). In this disease, the bone around your teeth is dissolved by bacteria. In severe cases, this can cause teeth to move. Severe periodontal disease in a young person may be a sign of another disease, such as diabetes.
Unusual tooth movement may also be caused by habits. Constantly biting on objects, such as pens, could cause your teeth to move.
The only way to find out exactly what is going on is to visit your dentist.