What Causes Outbreak Of Rashes In The Lower Part Of Body In A Child?

my grandson 3 y.o developed sudden onset of rash (not raised,no pusutles or drainge) first began over lower stomach then hips across buttocks then lower back, some spots on upper thighs, ankles/feet- had wrist swelling earlier- has gone down, now swelling of ankles and doesn t want to stand or walk dueto pain?