Why are my new brakes squealing in my 98 Chevy Malibu?

[Brielle'sMom]

I just had roters and brake pads replaced in my Malibu. They didn't "squeal" at first. Just started doing in 2 days after I had them done. Its not all the time. The "squeal/sqeak" when i first brake; when i brake harder it stops?
*** UPDATE; CERAMIC BRAKE PADS INSTALLED, NOT SEMI METALLIC