Some plants are sensitive to any changes in light, temperature, etc. I hadn't heard this about poinsettia, but it's certainly possible, especially if you're new apartment has less light than your old one. It does sound like you are watering it the right way by only watering when the soil feels dry; but as already mentioned, do be extra careful with overwatering right now since the plant has less leaves. Poinsettias don't like wet feet and are highly susceptible to root rot. They also do tend to drop their leaves very easily if stressed or disturbed too much - and during these times are even more sensitive to root rot.
It may be a compounded issue of the original move, then being placed too near the heater, and then being moved again (and if getting inadequate light, another whammy) - it just hasn't been able to stabilize with all of the changes. What are the conditions it is currently in (particularly lighting)? Poinsettias aren't really optimal houseplants, but they *can* live indoors with good care.
