My last cocker, Sophie, fainted at the groomers once. They brought her around with aromatherapy oils which they used to calm the dogs before they groomed them. She was about 12 at the time and although the groomers were lovely she had not been going to them very long and to be honest she didn't like going there. As I had her completely clipped I did it myself from then on. The vet said they would do a complete cardiac workover on her to see if there was a problem but as she hated the vets so much I didn't put her through it. It never happened again and I honestly think it was stress. She lived happily until she was 14, still walking up to 7 miles at the time, so I don't think there was anything wrong with her heart.
Hopefully it will never happen to your puppy again either.