I've noticed a big pigmentaion on my upper arm , it has a very light brown color and it' so random in its shape , what can it be ?

Need a picture: Very difficult to say without a picture to look at. It could range from mild normal pigmentation changes brought on by hormonal activity or sun exposure to congenital pigmented lesions which range, again, from benign to potentially dangerous dysplastic nevi to early melanomas (lentigo maligna melanoma as they used to be called)..any pigmented lesion should be checked by the appropriate physician.