Well, generally speaking, it is difficult to say what the real cause is, for there are some possibilities. In common, it can be because of Grand Papillary Conjunctivitis which is a very common type of eye infection may occur. When it occurs, it can just make the back of your eyes hurt. And in this situation, it can be dangerous, for it is very easy to transfer this infection from one eye to the other with your fingers so that's why the other one may have started hurting. By the way, strain in the muscles that surround the eyelids can lead to the hurt of your eyes too. So there are many reasons which can be the causes. Anyway, I will suggest you go and see the eye doctor, and he can give some advices.