Sudden change in behaviour. Advice please

[Marccharris]

We have two shorthairs who are sisters. They are about to be one in a few days.

The one sibling has started to be constantly demanding attention, with like a short purr or trill noise (very hard to describe) followed by a meow. It's pretty constant, but she does calm down.

Normally very active with her sister playing, but just noticed her hiss at her sibling instead of wanting to play.

She stops making the noise when one of us sits by her and when she makes the noises she's started being very low to the ground like literally sliding along her belly almost and rolling around a lot.

Doesn't seem to be in pain, all limbs are fine, she doesn't go outside.

Just wondering what it could be, or if she needs help?

Thanks.
Marc.

[fjm]

Has she been spayed? It sounds as if she could be coming into heat and calling.