No, she is not in pain, but she IS very frustrated. She is most likely on heat. This means she is ready to find a mate to make kittens.
It is a natural cycle which will wear off after a while, provided you keep her indoors and put up with her noise and restlessness. Make sure she does not get out or she will find a mate and you’ll soon end up with six or more kittens to feed and take care of!
It is better for her (and you!) to have her sterilised. This prevents her from having kittens and stops the urge she is having (which will happen every 3 months or so if the cat is not sterilised). For the same reason, it is important to sterilise male cats as well.
Sterilisation also prevents many reproductive organ illnesses common in cats. It is a swift operation and your vet will perform it under general anesthesia. She’ll be her spritely self again in a few days!