Wearing the old tray for few more days is never a bad idea. It is not unusual for the tray to not sit all the way for the first few days, which usually settles in after. You doctor also probably gave you special things to chew on which help engage the tray more. You should certainly ask your dentist what he wants you to do
Invisalign tray not fitting correctly
We normally do not see too many invisalign fit issues that early in treatment. If one side is loose, put the trays in and use the chewies that your dentist provided your with, In a few hours the trays should stay up and you should be able to proceed normally.