It is normal, very common situation. Pupil works just like the camera aperture, which functions to control the light into our eyes. If you are standing in the strong sunshine, your pupil will shrink to let less light into eyes. If you are standing in a dark environment, your pupil will dilate to let more light into eyes, so that you could see clearer. This is the way how pupil functioned. Sometimes strong emotion changes will cause the pupil dilated too, for example, angry, and extreme fear. It is common situation, so no need to worry about it.