Why does our ground beef smell like wet dog?

I started to cook tacos tonight with a package of ground beef bought fresh just a couple of weeks ago and kept frozen in our freezer. When I opened it, it was pink and didn't smell bad, and it was find after I defrosted it in the microwave...however, when I actually put it in the skillet and started cooking it, it smelled so badly of wet dog, I threw it all away! When my husband got home, we tried cooking another pound of meat, and it was fine until it had spent a little time in the skillet, then it too started stinking like a wet dog. It's so gross! Does this mean the meat is bad? This has never happened before! Thanks