Hi Doc. My Baby Is 1yr And 2mons Old He

Hi doc. My baby is 1yr and 2mons old he just well 3 weeks ago in fever cough and running nose. And i think its because of his immunization. But the fever takes 1 week . And yesterday may 31, he started to have fever again but its only 1day and now its okay . What might be the reason of his fever comes back? Im so worried about it.