my puppy keep scooting on her butt and whining or crying?

i need help my puppy who is about 4 months old she just started crying/whining and she scoots on her butt. Also she has started eating grass.
what do i do should i take her to a vet and also i am not sure if she got her shots she was given to me at 3 months old. HELP!!!!!
i picked up her tail because her tail has been down and her butt smells really bad and her butt opens a lttle bit .
what is wrong.?