iTunes Store: Associating a device or computer to your Apple ID - http://support.apple.com/kb/HT4627 - In connection with, "When you turn on iTunes Match or Automatic Downloads, or when you download past purchases on an iOS device or computer, that device or computer becomes associated with your Apple ID." "Once a device or computer is associated with your Apple ID, you cannot associate that device or computer with another Apple ID for 90 days."
It sounds like as part of doing things for your mother you associated the computer with her Apple ID. You then tried to change back to re-download your purchases to your computer from the Store because the first download was done to your phone. Read the section above very carefully because it outlines the specific instances where you hit this timer.
Transfer them from your phone to your computer, do not try to re-download them from the Store.
Right, I've read that multiple times, and I know that's exactly what happened, but thanks. I still have 60 days go to; I'm going to lose all of the songs I bought. At this point, twenty of the forty songs that were in my "recently added" playlist are now gone. I don't know how to retrieve them, and all I know is that they're on my phone, but not backed up. How, exactly, do I transfer them from my phone to my computer? I realize that I need to, but I don't know how.