Lump On Boxer Puppy's Throat/neck Grew To Golfball Size In One Day. What Could This Be? It's Right In The Center.

You do need to see your vet. Front or back? They are prone to tumors and cancer. Back could be an abscess and front could be something like strangles if the lump is really lymph node. Your vet will have a better idea after they are able to do an exam and talk to you about everything. First link is a link to your breed parent club with tons of Boxer info. Hope your baby is okay.
www.americanboxerclub.org
www.wildheirlabradorrescue.org  
members.tripod.com