You're not a bad mother. I don't know if you've seen, but I have seen at Babies R Us the alcohol test strips that test for alcohol in the breatmilk. If you are concerned about having alcohol in your milk, you might consider testing before feeding her.
Once the alcohol is out of your system its also out of your milk so no need to pump and dump and waste precious milk :) If you have enough milk to express the next feed and if she will drink from a bottle just do that to ease your mind :)
I take sip of beer here and there, no you are not a bad mom. Just drink right after you breast feed and limit your self to one drink. If you are not sure then just pump and dump plus give baby a bottle in between. And no drink and drive of course. Otherwise relax and enjoy :)