, forms deposits in the skin.
The second possibility is of fixed drug eruption. Were you taking any medicine before the symptoms appeared? FDE or fixed drug eruption may fade slowly and so may take a long time to go away. Although they resolve spontaneously but an oral antihistamine (e.g., hydroxyzine) and a topical corticosteroid may be needed.
I would suggest you to consult a dermatologist and get these possibilities evaluated. Treatment depends on the confirmation of diagnosis.
I hope it helps. Take care and please do keep me posted in case you have any additional doubts. Kind regards.