button mushrooms turned red while frying in butter
According to this article, button mushrooms may turn a pink color if bruised while being stored or handled. This sounds very much like what has happened to you, as you've stated that the mushrooms were old and not stored properly. This is not poisonous or bad to eat by any means, but I understand that one would not want to take the risk, especially when it comes to a fungi such as mushrooms.

For more information on storage, see the article provided above. A short excerpt on storage taken from that source:


  Storage - Agaricus bisporus will last longer in your refrigerator than
  many other mushrooms, usually around a week. It's better to store them
  in a paper bag rather than plastic. They'll last even longer if you
  take the time to put them in paper!