Stitches from umbilical hernia repair (5 months ago) were apparently left in my dog. Help?

Wow! I don't blame you for being mad. You should wait until you see your new vet and find out exactly what is going on. Many times the vets use stitches that dissolve and do not need to be removed. When you see your new vet, ask him what you need to do to resolve this, and who you can contact if the first vet was negligent.