R.S.
My son had this shot when he was an infant. He developed a fever as well and his leg swelled ridiculously like there was a softball under the injection site. The nure who administered the shot told me to give a cool compress and cool bath as well as tylenol. She took down the information to record to the VAERS (Vaccine Adverse Event Reporting System). His fever was gone within a day or two and the swelling in about 3-4 days. He didn't develop a rash however.
I hope your niece is felling better!