My cat yowls when I touch her tail and hind legs

Hello,

My cat is making angry meows (or is she in pain???) and hissing whenever we touch her tail and hind legs. Even just brushing them with a finger makes her bolt and yowl.

She is two years old and has been spayed.

Up until now, it wasn't too much of a worry, but as of today, she is also hissing when we pick her up. She doesn't seem to have any trouble getting around or jumping (at least, nothing has changed since we got her).

She enjoys the sun and spends her days and nights outdoors. Could this be a behavioural issue?