You may well need new rotors... but first, did you ensure the wheels were torqued properly (with a torque wrench, not a calibrated elbow
) when you did the job originally? Is there any way you could have got grease or some other type of contamination on the pads or rotors when you did the work? If there is any contamination, you could use some brake cleaner to try and remove it (be careful with the cleaner, if you use one of the aggressive ones).