Working out but not sore after heavy lifting, is this a bad sign?

Not necessarily.: You may have achieved a training effect from working out, where your muscle are now conditioned/acclamated to the stresses that you're putting on them.

It depends: It depends on what theory of resistance training you care to believe. I personally believe in the 'no pain, no gain' theory and i feel i did not put enough effort into the workout if I am not sore afterwards. I do not believe in the theory that your muscles somehow accommodate. To increase muscle size you have to break down existing muscle, hence the transient inflammation and pain.