Hmmm....if it were me, I'd be putting alot of pressure on the dentist to make me a new one, free of charge! After about 2 months after I got my new porcelain crown, I got an Astro Pop stuck to it. I had to work the sucker back and forth until it broke, 1 centimeter thick of hard candy, to get it off of my tooth. My crown (luckily, and amazingly) stayed on.
Now for your brand spankin new crown to come off, unless you've done something like i just described, I would most definately believe this is the dentists error, not yours. Possilbly the wrong cement, or even bad cement, not enough cement. Possibly the crown wasn't even made right to begin with. The doctor only knows! But I'd try to get him to replace the crown. Even if you only had to pay the lab fee's would be cheaper than putting out the whole cost yourself. Let us know what happens!