That is a great question and one that is best answered by your primary care doctor
or another medical care provider after they perform an appropriate history and physical examination. In general, based upon the limited history provided, it is difficult to know exactly what is the cause of this black spot under the skin on your toe but it does not sound from the limited history that this is serious. At the same time, because I cannot see the lesion or know your history, it is important that you are evaluated by a primary care doctor as to what the cause of this could potentially be. There are several causes for this including trauma, normal skin contours or more seriously there is a potential underlying pathology such as cancer. The only way to know is to be evaluated by your primary care doctor as he or she will best be able to know the cause of this. You may also consider being seen by a dermatologist
as they would be able to evaluate this skin lesion best and know what the cause of this could potentially be.