Why does my 5 month old chihuahuas ear droop?

When we first got him only 1 would droop but than both of them straigtened. Now, all the sudden one is droopy. I don't mind, I think he looks cute that way but is it a medical problem, should we bring him to the vet? I don't thinkhe is depressed because he plays with me alot. (we play tug a war and fetch ) We don't abuse him either, we give him food and water and take him out at least 3 times a day to use the bathroom and at least 1 walk.