Most bumps you find on a dog are what are called Fatty Tumors and those Fatty Tumors are not cancerous.
It's also possible that your dog could've bumped into something causing the bump.
Without knowing where the bump actually is it's a bit hard to say what the bump on your dog could actually be other than it being a Fatty Tumor or just a bump from an injury or it could even be a sore from something biting the dog.
If you're unsure of what caused the bump it wouldn't hurt to have a vet check it to be sure it's not a cancerous tumor but most likely it's just a fatty tumor or sore.