Based on the history and examination findings, the key differential diagnoses are:
1. PID
2. UTI
3. Endometritis
4. Appendicitis
Investigations:
1. Urine R/E and C/S
2. CBC, CRP, LFT
3. TVS pelvis to look for any pelvic collection/abscess
4. High vaginal swab for microscopy, culture and sensitivity
5. Treat empirically for PID with IV antibiotics like ceftriaxone and metronidazole pending culture reports. Admit for IV antibiotics.
6. Review in 48-72 hours for response to treatment. Consider laparoscopy if no improvement