Hi there.. this is contusion which is blood collection beneath skin in subcutaneous layer due to trauma or any bleeding disorders..but since you have only one then it's due to trauma only which is unnoticed.. usually in obese people such contusions happen with even mild injury which wouldn't even cause pain. Nothing needed to be done as it wil resolve on its own.. if you see multiple of such things appearing then it's cause of alarm as it may be due to some bleeding disorder then you need to consult with physician/hematologist.. headache may be due to fatigue, sleep deprivation, stress, the effects of few medications, the effects of recreational drugs, viral infections, common colds(most common cause of headache), head injury, teeth or sinus diseases, refractive error, any bleeding in brain or infection or tumors of brain, nerve compression in neck.. take Nsaids like paracetamol, diclofenac for a day or two if it still doesn't reduce then you need to consult with physician once..