A second flush of flowering is not uncommon. But you say "stem". Is it a green stem with a leaf or two, or just more flowering? If it is just more white filaments, don't worry. If it is a true stem, green with leaves, then the plant has reverted to vegetative growth, stopped floweing. Time to harvest, or if you can, return the plant to 12+ hours of darkness per day to re-initiate flowering.