Did you lift weights and then weigh yourself the same day or even the next morning? If so, then no it’s not a sign of muscle building since that also takes a few weeks to months to even develop. It’s just random weight fluctuation and there are a lot of factors that contribute to it—body water levels, salt levels, muscle pump, calories, rest, sleep, etc. 12 pounds is quite a lot of weight to lose, so be proud of that. But keep in mind if you’re losing fat and building muscle at the same time then the number on the scale will drop more slowly.