Has this happened to anyone? (I've only been riding for maybe 5 days and this appeared yesterday) I have a metal allergy but I'm pretty sure that's not related, and sort of needing suggestions on what I can do.
spider bite, scabies, bacteria...
start eliminating causes.
put helmet for a few days in a freezer to kill unwanted organisms, no joke
I don't think it is wise to freeze your helmet.
You will damage the glue for the liner parts and likely cause issues with the shell.
Wash it in baby shampoo and a touch of bleach then let dry properly...dry it out with a towel first, inside with towel do your best to remove as much water as possible then air dry with a fan.