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?

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.
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.