Have you been able to look and see if it's a tick? Sorry if that seems like a silly question, but I don't know if you've come across ticks before. If it isn't a tick, but you have them in your area, could it be where a tick has latched on, had it's feed off poor Pops and dropped off? When that happens there's very often a little lump that feels quite hard but usually disappears after a few days.
As piph has said, another possibility is a small injury from running into something or knocking against something.
Young dogs are also prone to harmless lumps called Hystiocytomas. If you do a search on here you'll find quite a few threads about them. They often disappear but are sometimes removed by the vet.
Try and have a better look when Pops is sleeping to see if it looks sore or scabby, but please don't get too worried about it as it's almost certain to be something simple and harmless. Having it checked by the vet is a good idea though
