According to your description, maybe it is because you are tired. As we know that when you have been reading, that means your eyes focused on one point. If it happens in a long time, it will make your eyes feel stressed. We can say it as eye strain. Then after you finished reading and want to look at other things, it will be hard for your eyes to be accustomed to the situation around. At this moment, they can not focus on easily. So you should be careful about it and just control the time of reading. For example, you can have some eye drops so that it can relieve the symptom.