Could she be older than 6 weeks? I remember my cat acting the same way before she was neutered..we were waiting for her to get to the proper age, but she started making chirruping noises and rubbing her bottom up against things ~ as well as rolling around on the floor rather seductively!
As she had been feral, we didn't know her actual age..just an estimated guess from the Vet. We phoned the Vet to ask his opinion and he told us to bring her down to the surgery immediately for the operation.