Welcome and thanks for posting your query to Healthcare Magic.
After reading your query the closest possibility of your problem appears to be "Urticaria". This is a condition caused by bursting of allergy producing cells known as "mast cells". As the chemical inside them is released causes the kind of rash which you mentioned.
There are many factors inducing the increased number and activity of these cells. The one factor as you noticed as constipation
can be aggravating factor not directly but as indirect factor causing stress.
Kindly follow the suggestions below:
-Apply over the counter calamine
lotion 3-4 times a day.
-You can do cold water compresses to reduce the intensity of itching.
-Do not scratch or rub the skin as it can lead to further aggravation.
-Avoid stress of any type, especially avoid constipation by eating fiber rich diet containing salads and fruits.
- Do yoga and meditation
-You can take oral anti histamine
or levocetrizine under medial advise.
Despite above measures if the problem is not getting better then it is better to consult a dermatologist
for best guidance.
Hope these informations will help.
"With good health wishes"
Dr Sanjay Kumar Kanodia
MD (Dermatology & STDs)