Easy. It is a fungal rot caused by either little air movement or water sitting on the leaf. If i were you i would cut the tip off, dust with some powdered sulphur or cinnammon, yes cinnammon (it has some good anti-bacterial/fungal properties) and improve air movement-add a fan to the grow area or open a window etc, and ensure the leaves dont have any water droplets on them for extended periods of time. You can do this by ensuring you water or mist the plant in early morning, because if you do it at night, it sits on the plant until late the next morning. By the sounds of it, you watered it then closed up the house? This would have contributed because of the lack of air movement. Phals usually stop growing when it gets below 10C i found.