Thanks for the query.
she might have developed a sebaceous cyst
which got infected,
Another possibility is a bacterial infection like furuncle
These are deep bacterial infections.
Treatment is with antibiotics, or surgical removal depending upon condition
Please meet a dermatologist
for exact diagnosis.
Let me know if you have any other doubt.
you can ask a direct question to me on this forum, following the below link.
Wishing you a good health.