Actually, it is normal behavior. Did you know Switzerland makes it illegal to own just one cat? The reasoning behind this is that animals need the contact of others of their own kind to be happy and healthy. I agree with this reasoning. It's not uncommon at all to have a kitty who is by herself become lonely. For this reason, I always recommend that a person own at least one cat. Having more than one cat encourages healthy bodies and behavior. It's shown that cats who live with other cats have fewer behavioral disorders due to anxiety, since they always have a companion to keep them company. They show fewer problems with aggression and shyness. They don't scratch, etc., out of boredom as often. And cats who live with other cats tend to stay in better physical condition, as they play together and keep active. Mentally, it's better for them, too. It's actually proven that solitary cats' brains may weigh LESS than cats who live with other cats for lack of mental stimulation!
You should talk to your vet about her behavior, though, because it's possible a physical ailment may be causing her behavior.
If it's just not feasible for you to add a companion for your little one, make sure she has plenty of toys to keep her entertained. I'd even suggest buying a video for her to watch on TV while you're gone. See
www.videocatnip.com and
www.kittyshow.com .
Your cat may settle into being an only cat, but it's not unusual to take several weeks.