I was in quite bad shape OR I was in a quite bad shape.
--- Should I put "a" before "quite"?
Is it because "shape" is uncountable?
We could talk about "the shape one is in", but we don't about "a shape that one is in", and the same thing goes for "health" as well.
In the concrete understanding of "shape", "shape" is countable: squares, triangles, circles. How many shapes do you see on the paper? We can count shapes but not health, so that's why it seems to me that "shape", as it refers to health or a condition, is not countable.
This is a good and interesting question. That's my take on it.
It could, but to tell you the truth, I would likely say "very bad shape" or just "bad shape".
Personally, I still wouldn't say "a bad shape" referring to one's health just as I wouldn't say "I was in a bad condition". There's no need to quantify "condition" or "shape" when they refer to someone's health. I suppose that's why it just doesn't sound like something I would say, nor does it sound familiar. As far as the pure technicality of whether or not it is correct in a structural way or a grammatical way - well, that's another story. And maybe another poster could reply here?