Suffering of high-grade fever 39.5°C most of time for 5 weeks, all tests normal, blind treatment of bacterial infection ineffective?

Suffering of high grade fever 39.5°C most of time for 5weeks

Suffering of high grade fever >39.5°C most of time for > 5weeks
She has no any signs or symptoms of infection or inflammation anywhere in her body , and all possible investigations done and all results within normal levels including : CBC, ESR, CRP, U/E/CR, LFT, TFT, BLOOD FILMS, LP, BLOOD CULTURES, URINE R/E, VIRAL SCREEN, EB VIRUS WIDAL TEST, Weil–Felix test , Brucella test, ABDOMINAL U/S, HIP JOINTS U/S, THYROID U/S, CHEST X-RAY, HIP JOINTS X-RAY, LUMBER VERTBERE MRI ( and some tests repeated many times )
She also received multiple antibiotic course as blind treatment including ( septren, cloxacillin, claforan, meronem, clarithromycin, procaine penicillin, ospen …… )
Clinically : except of her body temperature 39 -41°C despite paracetamol ( iv, orally and suppositories ) and ibuprofen syrups, she is stable , no complaint , no rash , no any significant symptom .
Note : 6 month ago she starred to gain weight and get obese , now her body weight = 30kg.
Female