I came on my period on the 5th my normal period is on the 7th but the fertility calendar said for me to expect my period on the 12th why did it come?

Many possibilities . If you have a short luteal phase that can account for what you describe. Certainly whenever your period is abnormal you have to double check that you aren't pregnant.
Normal cycles. Normal cycles come every 21-35 days. A period coming two days earlier than you expected is very common and normal.