i have had a poinsettia for a week and alreadt it has lost five leaves. why? I am not overwatering it and keep it away from extreme temperatures. Help

Perhaps it had a cold shock even before you bought it.

Yes I agree with Wagger, it may have been kept at too low a temperature before you bought it and may have been frosted.
The only thing to suggest is to either just enjoy what you have or take it back to the shop or nursery you bought it from. That's bad luck.