I completely understand your concern and I hope I am able to help you for the same.
You seem to have developed an allergic skin reaction.
I would advise you to use a mild steroid
like hydrocortisone cream
over the affected area twice a day for 4-5 days.
Use a sunscreen regularly before going out in the sun.
Once the reaction heals up , then you can start using a lightening agent like kojic acid
with vitamin C
twice a day till the pigmentation improves.
Hope it helps
Best wishes for a speedy recovery
Dr Geetika Paul