Time Machine and newly "fixed" computer

Hello,
I got my computer back from Apple about a week ago, and just realized that Time Machine hasn't been backing up since. Now the external drive is full - whereas it used to just replace it's own info (right?). Should I erase all the files that show up on the external in the finder window and do a new back-up?
Thank you!

 

Okay, the strange thing is that Time Machine thinks my external drive is full, but when I "get info" on the files on the external, they don't even take up half the space. What is going on???