Wait, how is it a doujin when it's by the author herself? It's clearly an extra if you ask me~ XD Also, been a very long time since we read Girlfriends by this author.
I was fooled too by doujin and at first I thought it was just someone who really had Morinaga Milk's art down pat.
But doujin means self-published and I guess this was. So "original doujin" even though you've debuted and worked with a half dozen magazines.