I don't think it's anything to really worry about. I've never had a cat suckle herself, but I've had a cat, about 6 months old as well, that would suckle one of our older boy cats. I think that it is just a comforting habit for her. Also, how old was she when she was taken from her mom? Because this could also be a result of her being separated from her mom too soon. Hope this helps