My sisters dog is ill can you help?

my sister has a dog she is a 7 year old border terrier cross lakeland terrier last week my sister s boyfriend accidentally stood on the dog and she yelped but was fine after, that was 3 days ago now, this morning she couldn't walk properly and she yelped when she did walk, my sister took her to the vets today at 11 am and was given meta-cam to give her once a day and was told to take her back on friday and today is monday, but a few hours later the dog got worse she started yelping when she moved and yelped when ever anyone went near her, i phoned the vet they said bring her straight back down so i did and was seen by a different vet who told me she had a spinal injury and gave her a stronger painkiller and i have to take her back tomorrow tuesday for xrays. is this really true? because why didn't it hurt her when she first got stood on? and why didn't the first vet tell us this?