Is she spayed? A cat in heat will frequently vocalize excessively (usually when you least want them to), and may be either unusually affectionate or aggressive, or alternate between the two. The heat meow/yowl is often quite distinctive, very hollow and pained sounding, accompanied by striking strange-looking postures as an invitation to mating...but if she's not spayed, she may well be in heat at 6 months. Heat is painful for cats, and too many "empty" heats can cause a uterine infection that is very dangerous, so it would be in the cat's best interest to set up a spay surgery as soon as her heat passes (in the meantime, keeping her closely confined indoors away from male cats to avoid pregnancy).
If she is spayed, and this is a new behavior, a call to the vet may be in order to rule out any physical causes. Once physical causes have been ruled out, start looking at anything that's changed recently that might be making her upset, insecure, or otherwise stressed.