First see what the vet has to say.
My cat has been doing something similar, although she does not have the pot belly. I took her in for blood work and nothing unusual was discovered aside from an existing kidney problem.
It could be excess stomach acid or constipation. They will sit that way when uncomfortable.
Prayers for you and your cat. It is terrible for us when they get sick.
[It could be excess stomach acid or constipation.]
It could be worms.