Why do my eyes dilate when I'm not on drugs?!

The simplest explanation could be that, because the nervous system controls the muscles of the irises, the response of the nervous system to different stimuli results in involuntary pupil dilation. It also could be that you simply are ultra-sensitive to light or changes in lighting. In some cases, pupil dilation is a common response to being poisoned by different kinds of plants and chemical poisons. Rather than take medical advice from a public forum, I strongly suggest that you seek the professional opinion of an ophthalmologist. An ophthalmologist (as opposed to an optometrist) is an eye M.D. - a medical doctor who is specialized in eye and vision care.