What is causing me to have these comprehension changes?

to see words I know, but instead see something different,for example:i'll be reading a sentence and one word in the sentence,usually more than a one syllable word,will appear as , ex : "The work we did was futile." The word futile could temporarily be processed by my brain as a" few tiles",until the real meaning kicks in. Another part of this is my brain is processing the change at a stop light from yellow to red to be from yellow to green, not processing the color just the change, This is very wierd and wondered if ANYONE has a clue for me? Thanks