Hello and welcome to the forum. Thanks for your question. I looked at your profile picture closely and do notice a dark line across your nose. Do you have allergies? Sometimes if someone has allergies and there is repeat pushing of the nose (wiping, itching), this type of line can form. The technical term is hyperpigmented horizontal nasal crease. As the nose is pushed upward repeatedly, sebum, an oil produced by skin glands, begins to accumulate on the bridge of the nose. After a while, the sebum begins to darken as it is exposed to oxygen. Sometimes they do fade on their own especially if a person is aware and stops rubbing or pushing on their nose. If you have allergies, perhaps antihistamines would help? You can try to lighten it as well if it doesn't go away. A combination of honey and lemon juice or just lemon juice applied is one home remedy. Also applying aloe or vitamin A cream to it twice a day may help.
If you don't have allergies, then do you have a history of hyperpigmentation in general?
A dermatologist would be best to guide you. Can you see one?