My 21 month old has low grade fever since thursday. today she started to shiver at one point and her fingernails got blue to a few minutes then normal?

Temp increase: Shivers and decrease blood flow to the extremities (blue fingers) are two ways babies make their temperatures increase. Your baby is fighting some infection, whether a virus or other, and the brain is telling the body "raise the body temperature". The body responds by shivering and decreasing blood flow to skin and extremities so less heat is lost and temp can increase to help body fight the germs.

Fever Can Cause Shiv: Even low grade fever can produce shivering, chills and sweating. Paleness of extremities often related to body's fever response that adjusts amount of blood flow to fingers, toes, etc. If "blueness" seen in fingers, toes, lips is associated with any shortness of breath or breathing difficulty- child should be seen by md immediately.