Mustard Seed

I've learned the disappointing way that grinding mustard seed doesn't produce dry mustard. Okay all you experienced chefs, you can stop laughing now. I ground some up with my mortar & pestle; it tasted awful. Was it low-quality seed? Or is mustard seed something else all-together?

Checking out Wikipedia it looks like I was on the right track. Gind the seed, mix with flour and water, get mustard. What about vinegar? There are lots of recipes on Epicurious that incorporate mustard seed. So now I'm thinking my mustard seeds are poor, old, and crappy.