Talk to Pediatrician on Fever And Crying

My niece is getting sick every months from the past 4 months. She got sick suddenly when she turned 11 months old, before this she used to get sick after 3 4 months. Now she's getting sick every month. I'm really worried about her. She has no problem. She just gets fever and there's no sore throat or ear infection. She gets fever and it gets better within 2 3 days. But I'm worried why is she having this every month or after 15 daya