it could be from the shots since it is so close to the injection area. Also, it could be a lipoma - fatty tumor -- although I don't know that they are that common in pups. Although I have read they are common in terriers -- the really weird thing is both of mine have one in the exact same place & they aren't related to each other, so it's not hereditary
Here's a website about them - I know there is a lot on the internet about htem
Lipomas, Or Fatty Lumps, In Dogs
I think I would talk to the vet though, because I bet it has more to do with the vaccine injustions