Loss of weight, less appetite, and small dark gressy poop

My pup has been not feeling well since I got her. She was with runny nose and sneezing. I immediately brought her to the vet and the vet had given me antibiotics for her. Finally, she was cured but now I'm worried. She has less appetite with puppy food and she's getting skinnier that I can touch her back ribs. Her poop was small and a bit dark and shiney. What should I do.

I'm sorry to say this, but you should go back to your vet - sooner, rather than later.  It can be absolutely normal to touch a pup's back ribs and it can also be normal for dogs to refuse food if full, so some people saying this about their pets will have completely normal animals and be over-interpreting when there is no problem.  However, in my opinion it isn't normal for a healthy dog to refuse food if the owners expectations are reasonable, and it sounds as though you need to know whether your dog is of a reasonable weight, or underweight.  At the lower extreme, this could be a sign of congenital liver diseases, kidney disease and other serious conditions.  Your vet will help to place your pup on this spectrum and recommend tests in order to help you to figure out what is wrong.