Over the years after getting pregnant my neck has been getting darker n darker & its not dirt what causes this...?

Skin changes: This is likely acanthosis nigricans -- a hyperpigmentation of the skin, usually found in skin folds (like the back of the neck). This may be associated with obesity or underlying endocrine disorder, so it is best to have your doctor take a look and order some basic blood tests to rule out diabetes or thyroid disorders. Exercise and attention to diet are often helpful.

Pigmentation changes: Pigmentation changes often occur during pregnancy due to hormonal shifts. Commonly affecting the face and neck, these areas of hyper pigmented or darkened skin typically fade with time but this "fading" can be sped along with certain bleaching skin treatments. Ask your dermatologist for more information.

Check for diabetes: Google acanthosis nigricans - this is associated with diabetes so if it resembles your discoloration see your doctor for an evaluation.