Hi welcome to Health care magic forum.
Thanks for calling H.C.M.Forum.
It appears that you have a fungal infection
, that is why it is spreading from armpits to the neck. You needs to be treated with antifungal agents for complete recovery, along with topical anti fungal agents.
If it is an allergy
it wont spread like this, but may appear in other places of the body.
I advise you to consult a dermatologist
for diagnosis and treatment.
Wishing for a quick and complete recovery.