I have this issue too, i have NEVER signed in with another apple ID unless i have a split personality i am unaware of, this is, as another person wrote just another example of poor service. Totally unacceotable, and its my **** music i paid for it, i will listen to it when i want!!! there are plenty of free software packages that do the same as i tunes match, none though that are quite as stupid as itunes match.
P.S. i asked apple for my money back, as i have been having this issue for 2 months and they did.
Apple used to be great for customer services, with great products, now they have crap service (help desk closing at 8pm?!!!) and their products are not worth the money either. But as long as people keep buying them, they will keep on not giving a crap about their customers.