One thing to bear in mind is that all Masters/Mistresses are different, we react differently depending on our personalities, just like every other human being on the planet. Personally, I would want my sub to confess to me that she had disobeyed my order. I shouldn't have to ask if she did what I told her, It is assumed that she followed my order...If she disobeyed me and I found out some way other than her telling me directly I would be disappointed and hurt. Disappointed that she did not follow my instructions, and hurt that she did not trust me or respect me enough to confess her transgression.

As I said this is just my opinion...every Dom/me will have their own take on the situation, but I'm going to advise that you tell your Master, don't make it the first thing that is said...get the hello's out of the way first but tell him. Good luck.