This reminded me of the first day I had my first yorkie. It was around 11:30 pm on a Saturday night and while petting him I noticed a tiny lump on his back toward his left shoulder. Of course I assumed the worst and got very upset. I was waiting until Monday to take him to my vet (I wasn't close to home) but there was a free vet visit so I called that number. I thought I would get a machine and ask if they would call me the next day but the vet picked up the phone. I could tell I woke him up but told him I was freaking out, I just got this puppy and it had this lump, ect. In a tired but extremely patient voice he said, "Is your puppy microchipped?"
He was and that is what the lump was, I had forgot about it and just panicked. I was so embarrassed but very relieved.
Could that be what it is?