What Causes Fever Post A Broken Femur?

my son (2years 6months) has a broken femur (left leg). break was 3 weeks ago, is back home and in plaster, he had a fever last night at which point i gave hime some ibuprofen + calpol (called tylenol in u.s) this got rid of the fever and he slept fine, he woke again at 5am with the same fever, again have folled the same steps and within the last hour his fever has subsided, is this common? is there anything i should look for?