Is it his rib cage???

I've had my Staffie puppy for two weeks now. Last week I noticed what appears to be a large lump under his front legs. On investigation it just feels like his rib cage but when you look at him it just looks like a large lump. We took him to the vet last week just to get a check up and she said he was a healthy dog. I didn't notice it until the next day and my boyfriend says it has always been there, its just I didn't notice it before. He seems very happy and it doesn't seem to bother him, I just wondered if this is normal. Any advice would be appreciated. Thanks