Why do I have dime size round hair less spot on my chen that just recently came.?

Bald spot. This is probably alopecia areata, which usually shows up as round patches of hair loss. The cause is unknown, but may be due to an autoimmune reaction, where the body attacks it's own cells- in this case hair follicles. For additional information see http://www.Ncbi.Nlm.Nih.Gov/pubmedhealth/pmh000242, and your dermatologist.