How did I lose intelligence?

A week ago exactly I read a chapter of my book chapter 4 of "Working Smarter" easily and took good notes. And 8 days ago I read chapter 3 of the same book no problem with and took notes. Today I couldn't get through chapter 5. It seemed like a sentence on each page I couldn't understand after I tried going over it many times. It felt almost too challenging for me tonight and I felt stupid. How could this be? Did I really get dumber?