Hello and welcome to HCM.
From your description of a single red lesion appearing suddenly with itching, it most commonly maybe an insect bite(ant/mosquito/bug).
The swelling might have increased after scratching or rubbing over the site subconsciously. Do check if any similar lesions are appearing elsewhere and also if any other symptoms such as fever are experienced.
If you are staying with someone look for any similar symptoms with them.
It is also possible that it is an
allergic reaction to a substance that you came in contact with, maybe clothing,or outdoor material.
Keep a check on its progress.
Apply a
calamine based lotion for the
redness, itching and burning 3-4 times a day.
Take antihistamine Levocet /
atarax twice a day for 2 days and then reduce to nightly for 5 days. If the rash is not going down satisfactorily then apply a
mometasone cream twice a day for 5 days along with the above treatment.
Watch for spread of the rash or new rashes appearing on the body.