Actually, I wouldn't worry that this is a sign that it didn't work. It could even be implantation bleeding. As long as there is only a small amount, I would just keep hoping :-)
The only time this happened to me after IVF, it turned out that I was pregnant with twins (I had spotting on days 11 and 12 post transfer).
Good luck,
Rachel Inbar
www.FertilityStories.com