There may be several reasons for rolling bumps under your dog’s skin:

Vaccine injection improper. If the method of injecting the vaccine is incorrect, it will affect the speed of vaccine absorption, causing the drug ingredients to accumulate locally, resulting in tumors.

Parasite infection. If you often run in the grass and do not clean your skin well, it will cause a large number of parasites to appear on the skin, causing damage to the skin and soft tissues and causing hard lumps.

Tumor. If you feel any unusual bumps or lumps, you should take your dog to the veterinarian immediately for a check-up. The tumors may be malignant and may have serious consequences for your dog's health if left untreated.

In addition, there are other causes such as lymphadenitis and lipoma. If you feel unwell, be sure to consult your doctor.