So I Was Looking Closely At My Eyes In The

So I was looking closely at my eyes in the mirror when I noticed that my pupils were quickly dilating and contracting. Well I know that light can influence how your pupils act but the light in my room was not changing and I was staring at exactly the same spot. So it can't be anything with the light right? I was just wondering if it could be symptom of something like too much stress. I am also very worried that it might be something more serious, because there is this thing like a mental illness that runs in the family. I tried to research that disease but i have no knowledge of what it is. So do you think you can help me.