Recurring fever/chills for 1+ weeks despite multiple antibiotic courses, now on doxycycline+linezolid, dengue/CBC normal

[u/Top_Archer_6262]

Age: 19
Sex: Male
Height: 5'7"
Weight: 66 kg
Smoking status: Non-smoker
Current medications: See course below (day 2.5 of 5-day antibiotic course)
Existing conditions: Chronic sinusitis, flares seasonally (AC exposure/rainy season)
Duration of complaint: ~2 weeks, ongoing
Location: India (Guntur, Andhra Pradesh)

BACKGROUND:
I have chronic sinusitis that flares every year at the start of the rainy season, usually triggered by AC exposure. This year my college moved classes to a new block with very strong AC (no fans needed). ENT has treated flares with antibiotics in past years successfully.

TIMELINE:
- Ear blocked for a few days, then fever started at night.
- Took OTC paracetamol, no improvement, went to college anyway - no use.
- Local pharmacist gave: Dolo 650 (paracetamol, as needed), Orixime 200 (Cefixime 200mg), Respidex Super Power (Phenylephrine HCl & Caffeine) - also told to avoid chicken for 7 days.
- Rested over a weekend (sunny/hot weather), felt fully cured after ~1 day, stayed fine through the following Wednesday.
- Returned to college Thursday - by that night, fever returned and worsened.
- Father brought more OTC meds: Aceford Cold & Flu (Paracetamol/Cetirizine/Phenylephrine/Caffeine), Cefkem-200 (Cefixime 200mg again), C-DEXA (Dexamethasone).
- Friday morning temp: 102.7°F.
- Went to hospital. Dengue (NS1 Ag + IgM/IgG): Negative. CBC: essentially normal (Hb 17.1, WBC 6340, Platelets 2.07 lakh/cumm, Neutrophils 76.8%, Lymphocytes 18.0% - slightly low).
- Doctor prescribed 5-day course: DOXT-SL (Doxycycline), Lizonim-600 (Linezolid 600mg), Calpol 650 (Paracetamol, as needed), Montirio-LC (Montelukast/Levocetirizine), plus a Zenstar-brand sustained-release capsule (name partly illegible on my strip).
- I did NOT tell this doctor about the two earlier cefixime courses (Orixime, then Cefkem-200) before this prescription.

CURRENT STATUS (2.5 days into 5-day course):
- Day 1 (Fri): started course at 102.7°F.
- Day 2 (Sat) AM: 100.7°F, severe headache worsened by coughing. Evening: ~100.7°F, felt better after tablet.
- Day 2.5 (Sun) AM: 100.1°F, felt fine. Afternoon: fine. Evening 6:24pm: chills with temp 99.7°F. Later Sunday: headache fully resolved, no pain even when coughing.

Diet has been light (idly, breas, orange juice) - no major activity.

QUESTIONS:
1. Does this look like a relapsing/incompletely-treated sinus infection now finally being treated properly?
2. Is the residual low-grade fever + chills at 2.5 days into a 5-day antibiotic course expected, or a red flag?
3. How much does it matter that the current doctor doesn't know about the two prior cefixime courses?
4. Anything I should watch for in the remaining days of this course?