It can also be pain. You should take her to the vet. She may have an impacted bowel, or a tumor, or something internal that you don't know about. Some cats have trouble with an accumulation of hair in their intestines and it prevents them from going... Does she use her litter box at least once a day? If not she may have a hairball blocking her intestines or bowels. The former could prevent her from getting nutrition, the second could prevent her eliminating toxic waste. Either one would be painful.
Maybe she just wants attention? Also, has she recently had kittens that were given away or did you used to have another cat? She might be calling out to her friend or kittens that are no longer there. My cat used to do that, it broke my heart. But she gets lots of attention and toys to keep her occupied. :)