Yes, contact lenses can cause iritis. Iritis means the iris of your eyes is inflamed. It can have the symptoms such as eye pain, blurred or decreased vision, sensitivity to light, red eye, tears and smaller pupil on affected eye. When you sleep with your contact lenses or wear your contact lenses more than 12 hours, it can cause iritis. If you don't treat iritis, inflammation in the eye can result in the permanent damage or even cause blindness at worst. So in order to avoid iritis, you shouldn't reuse contaminated or infected soft lenses, and don't use lens for over 8 hours. It is best to give up using contact lenses as your mother told you.