I can't imagine any dog of that age that should have any kind of a hard lump on it's abdomen. If he were my dog, I would be concerned enough to ask my vet about it.
Did the lump come up very suddenly? It may only be an insect bite and nothing to worry about. By the way, allergic reactions to such bites can be treated with benadryl (diphenhydramine ONLY - no decongestent included) at the rate of 1 mg per pound of the dog's weight. Check with your vet for more info if the dog has any underlying medical conditions that need to be considered.
If the lump came up slowly and has been present for some time, that makes it more worrisome, so go ahead and have your vet check it out. Better safe than sorry. :-)
As Jaybay suggested, I would have the lump checked out by your vet just to be sure, since it's not possible for anyone to really know what this is, especially without seeing the puppy, however I am going to go out on a limb here and say that I'll bet you will come back and tell us that the vet said it was his sternum. In some dogs it is more prominent than in others. If it was a soft lump in the middle of his belly I would say it was an umbilical hernia, but a hernia shouldn't be hard. Please let us know what the vet says. I am curious to know.
Hooray! I'm so glad you had it checked out and there's no problem. Never feel foolish about asking your vet (or anyone else for that matter) any question. No such thing as a dumb question, right? Glad to hear some good news here for a change. :-)
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