Hi,
My 6 yo has had a lump on his neck, slightly towards the back, just above his left shoulder for almost 2 years now. It varies in size, (its actually about pea sized right now, but has been up to marble sized). When we first discovered it, he was complaining that it hurt a little when he pressed on it, but that was his only complaint. The ped sent us to an ENT specialist, where I spent the $50 co-pay for literally 7 minutes of his time, and was told to watch it and come back in 6 months. (I'm still watching it but I havent gone back to the specialist, ped looks at it every year and doesnt seem concerned.) Apparently whatever it is is fairly common and not a big deal. I'd say don't worry about it just yet - get it checked out to be sure, but its probably nothing to worry about.
D.