Yes, you should let a vet have a look at the wound as soon as possible. It could be bacteria that grew inside the healing wound without you noticing until they started attacking the tissue now.
There's a slim chance that your dog could heal without medication, but this wound is clearly infected and painful for your pet. In the worst case the bacteria will open up even more of the wound and infect the blood stream, leading to a fatal sepsis.
The vet will probably clean the wound with a desinfectant and prescribe antibiotics for your dog. It's very important that you give your dog the antibiotics for the whole prescribed time (usually 7 - 10 days), even if the symptoms are gone after a few days.