There could be different reasons why the skin on your neck is darker. It’s probably not worrisome, but you should mention it to your health care provider. If you also have irregular periods, acne, extra hair or unwanted hair on your face or other parts of your body, weight gain or trouble losing weight, it may be caused from a hormone imbalance.
For example: Many girls who have PCOS (polycystic ovary syndrome) have higher levels of insulin in their blood. Higher levels of insulin from the pancreas can sometimes cause patches of darkened skin on the back of the neck, under the arms and on the inside of the thighs.
It’s ALWAYS best to get checked by your health care provider!