My 1 year old female (desexed) cat has had a sudden change of behaviour. She has gone from a very confident, relaxed cat who loved being outside, to a cat that won't go outside (unless someone is with her), lays around, but has 'panic attacks' where she runs around rapidly for about twenty - thirty seconds before sitting back down again and begins rapidly cleaning herself. She has also become very needy and feels the need to be perched on my shoulder (not the easiest thing to deal with!) and at night wants to sleep very close to my face - I feel as though she wants to feel my breath on her. She is still eating and using her litter tray, and has no visual signs of being attacked. There has been no changes within her environment. Any ideas?

It is breeding season, and I suspect there is a lot of kitty drama going on outside that a human would never notice. She may smell tomcat spray or hear fighting/mating noises from some distance away.
Encourage her to stay inside, and check to see if your vet carries Feliway, a calming cat hormone that comes as a spray or a diffuser. It might help her feel more safe from everything happening in the outdoor environment.