Why is my Time Machine backed up Documents and Pictures folders empty?

Right now I'm terrified. I backed up my imac with Time machine before wiping it to give to my Mom for Christmas. After I made a time machine backup I double checked all the important folders to confirm the data was there. Most important to me were the Photos and Documents folders. I may have gone into those folders using finder and manually added files to the folders by dragging and dropping. But I am 100% certain that I double checked (and maybe triple checked) the folders using my macbook pro to make sure I could access everything I need. Today when I needed something from those old folders, the y are empty and when I "get info" it says the folders are zero kb. Please please tell me how this happened and that there is a way to get these files back!