Hair dye while pregnant?

Before you give me grief, I didn't know I was pregnant when I did this. 
Right before I found out I was pregnant, I had tried to dye my hair a dark brown over very light blonde. Simple enough right? I've done it before and the colour has always worked. Well it turned out a super light ash brown colour that was very close to blonde, the dye didn't take. Is it actually possible this was because I was pregnant? I've heard women say it's because of the hormones? Has this happened to you?
Wouldn't the fact that it was light actually take the colour more? That's just my experience with hair dye though!