Kitten is acting strange?

Question

I have a 7 month old kitten that's been spayed and has all her vaccinations. She has this red string of hers that she puts in her mouth all the time and walks around the house with it. Then she starts meowing and jumps on the couch. When shes on the couch she starts kneading the couch and the pillows with the string still in her mouth. Then sometimes she even starts meowing and walks away. What does this mean?

Answer 1

She is mimicking the behaviour of cats when they catch prey.

So, basically, she is pretending/playing.  She is pretending that the string is a mouse and she caught it -- and is bringing it to you for your praise.  

She is playing a game.  Totally normal and funny.

My cat does it with my ponytail holders.

Answer 2

She's just really happy.  Our cats each have their own favorite toys that they repeatedly "kill" and drag around.  Yours is probably kneading the couch because she's supremely happy and content.