Thanks for your query.
Fever is not due to constipation
per se . In view of normal physical check she might be just having a viral fever
but in this senario urine infection is also quite possible and it is a good idea to get a clean catch urine checked by dipstick and culture. Constipation is quite common in any illness associated with decreased oral intake. Continue giving her fever medicine as required & increase fluid intake .Also can try stool softeners like duphalac
for few days .
Hope you find the information useful.