My baby got vaccinated yesterday and she had fever. Why did she have fever?

My son usually has a slight temperature the day after his vaccine but it usually goes away on its own. The nurse will usually ask you to monitor the child's fever for the next 2 or 3 days as most babies have a reaction to vaccinations. Usually at the polyclinic, you'd be given medicine in case your child develops a fever. I personally just use a fever patch as I rather not get my son use to taking medicine just for a slight fever. However, if your child is runnng a high fever please bring your child to the doctor.