You would need to see a physician to get an answer to this. Several disorders that effect cognitive processes can cause people to have problems organizing their sentences, concentrating on and understanding what others are saying. etc. The disorders I know that can cause this are chronic fatigue syndrome and fibromygalgia. Toxin exposure can cause this, some medications can and I'm sure there are many other possibilities.
In chronic fatigue syndrome these types of problems are called 'brain-fog' and in fibromyalgia they're called 'fibro-fog'. If you're experiencing increased fatigue, high pain levels, problems with sleep and concentration you might want to look into these disorders.