You're speaking to Dr. Michael Salkin. Welcome to JustAnswer. I'm currently typing up my reply.
I'm sorry to hear of this with Coco. To answer you directly, yes, in most cases such panting represents involution of her placental sites - changes occurring in her overly sensitive uterus. I can't rule out the unlikely possibility that she's back in labor because a pup is very late in coming, however. Excessive panting lasting over 24 hours should prompt ultrasound of her abdomen to better understand her uterine health and possible dystocia (difficult birth). Please respond with further questions or concerns if you wish.