Worried For A Little One

I have a puppy here who is four weeks old on Monday, when he was born I noticed he had a sort of 'water filled lump' on his neck I took him to the vet to have that checked out and he seemed to think it may have been an injury he sustained from Mum possibly as he was being born (although she was far from being rough with them - infact quite the opposite)
Anyways I took him back on Tuesday as I was starting to get very worried about the little one as I noticed that the lump seemed to be hardening and also seems to be growing with him. Vet this time confirmed he wasnt quite sure what it was but he would have to have surgery when he is around 4 months old to get the lump removed, the pup whom I've nicknamed 'Buba' has not been rejected by Mum and has fed extremely well. What I am worried about is that he may have a cancerous tumour, how possible is this in a young pup & is it common?
Our vet although very good refused to commit himself as to what it may be - the only positive thing that was said it that the lump didn't seem to be attached to anything other than a tiny bit he couldn't pick up on as poor little 'Buba' screamed when he tried to proporly get a 'feel' on that particular area. So obviously where our vet was trying to feel was painful, but the lump itself is not. I try not to prod and poke it too much, but when I have he has'nt indicated that he is in pain. As I said he seems to be doing well, hes growing, thriving and seems to be a happy little puppy other than the 'lump'. Of course until he's reached an age that it is operable I won't know what it it until it's been taken out - all I am wanting to do at this stage is pick a few brains to see if anyone has experienced a puppy with a simular problem??

if pup is doing well then i would say don't panic ....its impossible to know what it is until its removed until then enjoy him like any other puppy .its prob nothing

Thanks Inca, although its hard not to I'm a bit of a worry wart he's back at the vets in four weeks time for the first set of his vaccinations and a check on the lump - then when he's four months old it will be time for his operation. I'll keep you updated on his progress

Thanks for the advice it has helped to put my mind at rest a little