Long low grade fever, infection?

For the last 7 days, our LO 1 year old had been in and out ER 3 times, bunch of tests, currently checkin children hospital, the cause is still unknown. Anyone experience these?

Symptoms:

- common cold/flu symptom

- low grade fever (98 - 104, on/off based on the ibuprofen/Tylenol)

- (odd symptom) pounding when try turning to left side, feel ok to sit lean back at some certain angles, does not want to stand at all (even when low fever and happily eating/drinking)

What we have tested so far:

- overall body (no bruise, no bleeding, no rash on skin. Normal)

- covid (negative)

- ear infection (negative)

- lung xray for pneumonia (negative)

- left leg xray for bone fracture (negative)

- join movement check (right side seems fine, left side a little tense)

- ultrasound hip join for inflammation (negative)

- mri scan (soon)

Dr thinks it is infection and try to scan everywhere for either virus or bacteria.