Dog suddenly has large lump on back of neck?

Posted 22nd Dec 2016
It just seemed to appear one day. His collar was put on earlier and there's a large lump (roughly 3-4cm diameter and 1cm thick, moveable and im pretty sure it wasn't there yesterday.) He's a 9 month old springer/lab. Any ideas what it might be?

He recently was castrated and on rimadyl if that's relevant. Other than that he's bolting around the house like an energetic puppy and is wolfing his food down so seems healthy enough.

I'll be taking him to the vets tomorrow but does it sound like something anyone has heard of.
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