My 8 mth old (7 months corrected age) low grade fever 38.2-5 c for 3 days. Cold related. Tempra (acetaminophen) helps. She is fussy and lethargic. Doctor time?

Yes. Lethargy is a concerning symptom when a child is sick. The child may be dehydrated or have a worsening infection. Seek medical care as soon as possible to prevent any further complications. Focus on hydration with pedialyte also.