I am sorry without examination it is not possible to comment on the cause of the marks and the treatment that would benefit you most. There are multiple causes for hyperpigmentation. Sun tan, overweight, pigmentation disorders like acanthosis nigricans or melasma, lichen planus pigmentosus, nutritional deficiency etc are some conditions. The batches of pigmentation need to be examined and assessed to know the cause. Treating it without knowing the underlying cause will offer incomplete and temporary relief; also it will keep rebounding. Get an online consult or meet a dermatologist directly for a proper evaluation of the condition. Hope this was helpful.