My puppy struggles to breathe, won't nurse, and is constipated. Help!

Oh no. Breathing issue in a newborn are considered critical. Take a pediatric bulb syringe and suction out the nose and throat area for any residual liquid. I suspect that your puppy may have gotten fluid in the lungs. There could be a congenital defect (i.e. cleft palate) that could have caused this or it could have been accidentally done by aggressive feeding from the mother where milk went down the trachea instead of the esophagus. It is critical to take puppy to your ER vet if he continues to having difficulty breathing after you suction everything out well. He may need oxygen support (if tongue is blue or not oxygenating well) and antibiotics if he is found to have aspiration pneumonia.

Constipation is also a concern but doesn't take precedence over the breathing problems. Please get puppy help asap because he is considered to be in critical condition and in need of medical assistance.