It could be a fat tumor. My dog has several all over her body. They are not malignant, and only pose a problem if they interfere with the dog mobility. In that case they can be surgically removed. Hopefully that's all it is. But you should have a vet take a look just to be on the safe side.
This may be a Zeist.
My dog is 5 years old and I found a lump on her upper side, on the left leg, what could it be?