Sometimes it swells a bit and feels painful

Hello. By observing the photo you uploaded, we can see the swollen lump is close to the dog’s mammary glands, so consider diseases such as mammary hyperplasia, mastitis, or mammary tumors. When a dog has mammary hyperplasia, the glands usually become hot, hard, swollen, and painful, and this situation can usually be treated with antibiotics.

If it is a mammary tumor, there are usually irregular nodules. It is recommended that the owner take the dog to a veterinary clinic promptly for examination and adopt targeted treatment measures based on the cause.