Great. Thanks for the additional information. I really appreciate it.
Dogs use their tails to express many things, so there could be a multitude of reasons why she's got a sad tail, but because this came on so suddenly, I'm prone to thinking it could be a case of 'limber tail'...or as it's called in Northern Minnesota, "Lab Tail".
We see many dogs with these flaccid, painful tails and although the majority of the time, we see it on Labs, there isn't really a breed restriction on what breed will get it. I've seen everything from Chihuahuas to Springers with Lab Tail.
You can find more information on the problem here:
As a general rule, if this is 'lab tail', at our practice we treat with rest and anti-inflammatories, so you may want to give your vet a call in the morning and just ask if they'd send out pain medications for her. Something like Rimadyl or Metacam (both of which are NSAIDS...like Aspirin) are good options.
I hope this helps.