If he is vomiting dark material this may be digested blood from an ulcer or other cause of intestinal/stomach bleeding. Because of that if you can reach the treating vet today I would suggest you call them for a consult.
Novox can cause intestinal bleeding resulting in weakness and lack of appetite.
It can also impact liver and kidneys causing illness. Its a great pain med but has side effects in some dogs. Typically blood work is done before prescribing it and is followed afterwards too.
Panting is a sign of pain or distress or difficulty in breathing which can in turn be due to anemia from blood loss.
If your dog appears dehydrated (see if a pinch of skin over the shoulder blade stays stuck in a tent form or flows back down normally when released), has a rectal temp over 103.5F (normal is 100.5-102.5 Fahrenheit or 38 - 39.2 Celsius ), has gums or tongue that look white, blue, gray or yellow tinted where they are normally pink, has a slow color refill if you press on a pink area of gum or tongue, is having difficulty breathing, or is becoming more lethargic and non responsive to you as time goes on, or more ill seeming, then you may want to get the dog in to see an emergency vet if youre vet is not available.
This site may help you locate an emergency vet if you need one
http://www.vetlocator.com/hotline.php
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics