Hello,
It can be due to tinea capitis or fungal infection of scalp. It is contagious and can spread direct contact with someone who has the condition, or if you touch contaminated items such as combs, hats, clothing, or similar surfaces. Diagnosis is by clinical examination of the scalp as well as a skin lesion biopsy with microscopic examination.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
A circumscribed bald spot, such as you describe, can be caused by hypothyroidism. Do you have symptoms other than the bald patch? Do you suffer from the cold?