I Have A 3-year Old Little Girl Who Has Had

I have a 3-year old little girl who has had a fever of 101+, about once a month. They are typically not associated with other symptoms. She is typically just cranky, has decreased appetite, sometimes complains of stomach pain and foot pain. Energy level is sometimes affected, but not always. The first two fevers lasted 7 days and 10 days. This third one started with a cold/virus, but those symptoms have cleared and the fever remains...now she has had the fever for about 3 weeks. Her doctor has previously just used a "diagnosis" of fever of unknown origin. Blood and urine testing at the end of the second bout of fever yielded no unusual results.
When should I begin to worry about these recurrent fevers? Should I seek a second opinion from a different pediatrician?