Why is the hair above my upper lip suddenly darker?

Sure it is your hair. Make sure it is the hair and not the skin. I bet it is this skin and you have something called melasma.If so, this can be treated with the use of the chain creams and the constant use of sunscreen. Don't forget a broad-brimmed hat in the summer.
Hormones. Some women are prone to extra hair, which can be effected by hormones, pregnancy, polycsticovaet , etc. This area is easily treated with hair removal laser.