Hi...... . Information regarding duration , progression, associated symptoms like itching ; burning n others is not provided. As per details provided It could be due to photosensitivity . Other possible causes could be melasma,Pigmentation secondary to drug reaction, post inflammatory hyperpigmentation, lpp, ect;Most of these conditions respond better to chemical peeling, regular use of sunscreens, antioxidants topically n orally. Other treatment options include lasers, mesotherapies ect.consult a dermatologist for appropriate treatment. Hope this ll help you. Have a good day