Hello, and welcome!
I'm sorry Tilly's new behavior has you worried. It doesn't sound like anything concerning. Many times, cats will chase shadows because they think they are prey or other living things that they can't catch. If a candle is burning, it's making shadows that are 'dancing' on the ceiling. If the candle is not lit, the lighting in the room may cause certain objects to cast shadows.
There's also a possibility that there IS a flying bug or other type of insect on the ceiling that she can see, but you can't.
As long as Tilly is eating, drinking, eliminating and active as usual, I would not be overly concerned with this new behavior, but if it continues or she seems obsessed with something on the ceiling, you can always have her evaluated in person by her vet to check her vision and reflexes and make sure all is well, medically.
I hope all will be well with your furry girl!
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