Why Does My Boxer Have A Swollen Tail?

I would see the vet. It could be a bite or something. Is your pups tail docked or not? If it's longer there is a greater chance of being broken. It's probably more common than you think and not generally as expensive or as complicated as other breaks but nerve damage is a concern and particularly as you seem to be noticing a lot of swelling.
If it's short nerve damage is also a concern but may not be from a break (did a vet do the dock on the newborn pup) and infection or something may be a concern and especially if your pups tail is very short and close for some reason. I would go in for an exam, it's inexpensive and you get an idea and where you might go from there. You may not have to do much beyond that and you can make decisions after the exam on what to do. If it's a bite or infection that suddenly caused that you don't usually want to wait too long.
Be careful, if it's painful (less pain can indicate mroe concern about the nerves) your pup may bite. It doesn't matter who you are when they hurt, it's a natural reaction. Keep your face away and out of reach at all times in case your pup does react. Most people would rather be bitten somewhere else if they are going to be bit so it's standard even during proper restraint and handling to limit facial damage in case of a bite.
Hope it's very minor and your pup is okay.
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