Why do the words sometimes switch when I read them at that moment and look unreal?

When i read words, I can read them, spell anything, and write anything. I'm a writer. But, sometimes I will read I felt good, and felt will be 'left, but then when I look back it will be felt. The words just look wrong, and i am not sure if they are spelled correctly, when I know they are. I feel like I am just reading the words as symbols. Do you know how I can stop it?