Want advice about my injured dog?

hey ther ive got a 11 year old blue heeler and for the last day and a bit he's been walking really slowly with his lower back kind of arc-ed out, also when i even gently touch his lower back or tail(he has a short tail, one of those stubb things) he yelps and is in obvious pain, im taking him to the vet tomorrow morning but id like to know as much as possible now