1.5 yr girl has a fever around 38.5 accompined with cold cyanosed extremities when she takes anti pyretic drug everything become ok why?

Several possibilitie: There are several possible answers. It could be secondary to an infection that caused spasm in the blood vessels; it could have been because of an allergic hyperpyrexia; it could be because of an inflammatory condition; it could have been low blood pressure secondary to the fever. If it happens again, she needs to be seen by a doctor.

Be seen: Fever and cyanosis should prompt you to bring her to her doctor for evaluation. The fact that it improves with reduction of her fever is a good sign, but she still should be checked for infection, such as influenza.