There are a number of reasons why this can happen. You may be developing a cold, or there may be debris or deposits on the lens, or a nick in one of the edges. If you haven't cleaned your lenses properly, there may be some deposits left behind, so clean them again. If your lenses have been cleaned, start by rinsing the lenses thoroughly with saline or an all-in-one cleaning solution to remove debris (never use water or solutions that are not designed for rinsing). Examine the lens carefully for nicks and tears; if it is damaged, throw it away. Never continue to wear a lens that is uncomfortable or irritates your eye. Consult your optician instead.