Why does my dog jump at nothing? (scared)

The dog keeps randomly jumping at Shadows, and I don't mean chasing shadows, I mean jumping at them.

It's like the feeling you get when you're walking down the corner and somebody jumps out and scares the bejesus out of you, that's the look she has on her which makes this little jump.

It's not like a Tremor or muscle spasm, there could be nothing around her at all sitting calmly relaxed and then all the sudden jump at nothing!
She could be standing in the doorway and totally freak out totally at random!

She's not abused, she is fed and watered every day and has a healthy safe home, she has her own little spaces and I'm home with her all day so I can't imagine separation anxiety (when do leave her alone on the house she is totally okay, no barking/damage/whining/etc ).

I'm curious as to what the heck this is?

Some dogs just get scared easy I think, maybe google things about how to prevent your dog from getting startled so easy.