I have a cat who is about a year and a half old. Sometimes when she's
curled up on my lap, purring away, she trembles. It's the kind of
trembling a cat does when scared, like at the vet's.

Is this a sign of a problem or is it just her quirk? It only seems to
happen when she's really happy and purring madly. When she's sound
asleep it doesn't happen.