For the past several days, I have noticed that every time I open Spotlight, it keeps on indexing my hard drive. I know indexing can take a while, but it seems to be taking days. Even when it seems it's about to finish indexing, it starts over again. I tried to have my Mac build the index from scratch by adding it to and then removing it from the privacy section in the system preferences. This hasn't seemed to work either. I've run first aid on disk utility and the hard drive appears to be in good health. Any ideas on a cause or fix? I run the latest version of El Capitan on a 2013 retina macbook pro with a 512 GB SSD.