Hi, Thanks for query.
There are 2 possibilities.
1, due to allergy
to metal as your doctor told as rash
spreads to neck and face.
2,after piercing the belly button you might got itching due to healing,scratch marks became infected and bumps are appeared.
take antihistamine medicine like Cetrizine
,apply antibiotic,antihistamine cream on the affected part.
Ok and bye.