Is it normal if my baby got some rashes after his fever ? should i worry ?

Rash after fever: It sounds like your child had a harmless virus which causes roseola where a child runs a fever for three days, the fever breaks, and the child breaks out in a rash. The condition usually affects children under two years of age.

"After the fever...": ...Is better than "during the fever." if the rash and the fever are not there at the same time, roseola is the most likely diagnosis, and it is basically harmless.