Hey there...
This appears to the naked eye and on first glance to be the reaction of something we allude to as Fixed Drug Eruption.
This does not mean it is related to drugs but can be any chemical or food colouring or addivite you are Allergic to.
Need to isolate this and Stop 🛑 it permanently.
A more detailed history will be needed and perhaps the best recommendation will be for you to consult Online via this App's INSTANT ONLINE Consultation 🙏
Can then work out what the offending agent is and prescribe a cream to remove the pigmentation.
Health Tips
Avoid OTC products, home remedies, self medication and Google suggestions 🙏
Have to isolate and stop any offending agent.. common are pain killers eg NSAID drugs.