cat is arching back and inactive, laying down

i have a domestic shorthair cat named cat, shes about 9 months old, and i noticed this morning that she was laying on the floor acting kind of strange, she was just laying on the floor meowing and purring and moving around a little bit, then she got up and i told some of the other people in my house to look at her and they thought it was strange too, she is currently lying on the floor, she arches her back kind of like it was something of a stomach problem, and she doesnt really act like shes in critical, sharp pain, but its definitely obvious that somethings wrong. It's really rather sudden, we have 2 other cats that live in the house and they are both older. the other cats are acting fine. Cat generally gets along with both of the other cats. Note that she isnt totally unable to walk.
does anyone have any idea what the problem might be?
-Xx

I'm going to move this to the Health and Nutrition forum, but I very strongly suggest that you take your cat to a vet ASAP.

Arching the back and meowing and purring a lot makes it sound like she might be in heat if she's not spayed.

Yep sounds like heat to me! But a vet can confirm this. She needs to be spayed ASAP!