Strange Blurry Vision problem. Reading?

I tested positive for type 2 diabetes but my last reading was 142 ( Random).For past 4-5 days my vision is disturbed. I am myopic and use glasses. I never had problem of reading with glasses. Now strangely my focus of nearby objects with glasses on is blurred. When I put off glasses, I can see things crisp and bright. Can't understand why the glasses which has no power for reading , cannot be used for reading now. Consulted an eye specialist, he checked eyes well but upon knowing I am diabetic, just said this may be because of diabetes! Even he could not clearly explain why my glasses are causing problem in reading now when they have no power for reading? Vision sometimes corrects automatically and I can see both far and nearby with glasses. This is causing lot of trouble for me.