Don't worry, it is normal. In fact, anyone of us have similar problems. No one have exactly equally balanced face, hands or foots etc. You can look at your picture. If the two eyes have no obvious different, it is ok to have one eyes look higher than the other. Even if some famous stars don't have exactly equally balanced face. So, you needn't care about it. Just be confidence.
Well, how weird it sounds! I don' t think this is common. Maybe you just get somewhere hurt so that you get this. You might as well think of these days carefully to find out what had happened to you. If this just happens for no reason, then I can only suggest you to go to your eye doctor. I know a guy who used to meet this situation but he changed that through a surgery. By the way, are there any problems with your eyesight so far? If not, then you may not worry too much.