I've had issues with getting the alternator belt snug enough the first time around. The tensioner for that belt can be annoying. If you didnt have the problem before, its more than likely just a matter of confirming all is tight (or even lined up) again. Belts will also stretch a little after they are first installed and run for the first time.
Please dont thing that I'm saying you didnt do it right the first time or anything like that. Its just the nature of the beast. Double checking couldnt hurt. Funny it would only squeal @ night though.

Also, If you still have the reciept or the holder deals that the belts came in...confirm that they were the right ones on the net. Those Adavance Auto Parts Guys have been known to drop the ball on replacement items like these.