What do you want from me?!

Fellow cat owners, lovers, aficionados...help me out.

My little cat Amalia (American shorthair--6 months old) is constantly meowing and rubbing up against me, appearing distressed. She usually purrs, but the meow sounds kind of sad or panicky. She's spayed, so she's not in heat (unless that happens no matter what--I'm not knowledgeable about that sort of thing). Sometimes she will go into another room and meow loudly; but as soon as you approach her, she runs out of the room in a panic. Other times I will be sitting at my desk and Amalia is trying to jump on my shoulder or back. I give her attention, like petting or scratching her head constantly (well, it feels constantly), and the meowing stops for a little. Or I'll hold her under my arm like a football, carry her around while I do this or that, and she is all quiet. But as soon as I put her down or turn away, she's meowing again.

Do I have a needy baby cat or is there something I'm ignoring? I keep her food and water bowls filled; the water is filtered and changed twice a day. We clean the litter box twice a day too. Her sister (American longhair) is nothing like her; she is usually sleeping somewhere, eating, and plays a little but is generally quiet.