Apartment dweller desperately needing help

[teenytiny]

Hello!
I'm new here but have been lurking for a couple of months-- the length of time that I've had my puppy.

I have a beautiful four and a half month old girl who, beginning about a month ago, barks almost nearly non-stop from the moment that I wake up until I go to sleep (around 3 am). The only exceptions are when there is nobody home (if we go in the hall and she thinks she's alone the barking stops immediately), while she eats, and when we're running. She even sometimes barks when she's being held! I don't think that she's in any pain-- she's been to the vet in the last week, and she moves very well and quickly. Her appetite is good.

My girl used to get about a two mile walk a day, but in an effort to tire her out it has turned into a two mile sprint-- difficult for me at times, but seemingly effortless for her.

I bought one of those ultra-sonic bark sensors that supposedly will annoy her into stopping, but I think that it might actually be encouraging her!
I really, really love this little girl, but I am terrified that we will be evicted if the noise doesn't stop.

Any advice for the desperate?
Thank you!!!