Thanks for your query.
Your daughter appears to have developed an abscess
(collection of pus caused by infection).
Sometimes such abscess have deeper connection & because it is located in midline & just in or above buttock line it might be reaching up to underlying back bone or spinal cord
She definitely needs treatment by antibiotics & perhaps exploration to see the exact extent of pus & deeper connection if any.
Please see a doctor soon.
Wishing her a speedy recovery.