It is quite abnormal. As I know, nearsightedness may not happen in a sudden, but once happened it will last forever unless you do the lasik surgery. See an ophthalmologist as soon as possible. Maybe there is something wrong with your retina. Confirm this with your doctor.
If you don't have vision problems before,it can't be nearsightedness. It is normal that you feel blurry and dark after a long time reading.You just need to relax and do some eye exercise ,it would be fine.