The most common causes of such prolonged rashes are:
Allergy / Atopic dermatitis: It consists of a red rash or bumps which can be found anywhere in the body. Cause is due to an allergen which is recurrent in the environment.
Contact dermatitis: this is a similar type of allergic reaction to something which is in direct contact with the skin, like nickel found in jewelry or watch straps, particular brands of cosmetics, etc.
Scabies: scabies is due to infestation by mites. You need antiscabies treatment for this.
Granuloma annulare: this causes recurrent bumps which are often in a ring formation, cause is unknown.
Lichen planus: this gives rise to itchy purple or reddish coloured flat bumps, cause is believed to be immunological.
Psoriasis: gives rise to itchy red rash associated with silvery scale like patches. cause is believed to be immunological.
Autoimmune conditions: like scleroderma, psoriasis, dermatomyositis, epidermolysis bullosa, and bullous pemphigoid
Skin infections: like impetigo or erysipelas.
For you the most likely is Allergy/Atopic dermatitis. This is often provoked by exposure to some allergen. You need to pay careful attention to your environment to see if you can find out any allergen which could be the cause. You may be having prolonged exposure to an allergen.
It could also be related to certain medicines, foods, suppments, etc. Check out whether there is any medication associated with the rash. Also try to recall any associated foods, specially foods like nuts, some new types of meat, fish, etc associated with the rash. You could be having a food allergy. You should also watch out for any soaps or detergents. Also try to recall any activities associated with this, like hiking to some place, cleaning the house or some area of the house, some hobby like wood working etc which coincide with the rashes.
If you find anything which could have caused the allergy, then you have to avoid it. However, quite as often, no definite cause of such allergic reactions can be found, do not worry if this is so. Most of the time, cause for Atopic dermatitis is not detected.
You should take an antihistaminic like Cap. Claritin (OTC) one capsule daily. Also apply 1% Hydrocortisone ointment. This will get rid of the rash.
However, if the rashes keep recurring, you should see a doctor, preferably a dermatologist. All the other causes I mentioned need to be excluded.
Hope I have been able to clarify the situation for you and helped in resolving your query.
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Dr. P. Jyoti