Suddenly lot of small pimple can be due to prednisolone /methylprednisolone/deflazacort ,etc that could have been started for your Rheumatoid arthritis. The Itching will need an evaluation and examination to better ascertain cause. The darkening that you are referring to can be from nutritional or even separate entities - peri oral dermatitis for mouth, stress induced for under-eyes. The small pimples (if drug induced) - is not an allergy but a side effect and would require examination and confirmation before advising further treatment. Your location says faridabad, I would recommend meeting a dermatologist or you can consult online with me as an examination would be required before commenting on the treatment. As fine examination would be needed - meeting a dermatologist in your area would be my suggestion.