Miss Sabrina got so scared last night

I don’t know what woke me up last night.   Miss Sabrina was sleeping down by Bob’s feet and he moved his leg and she jumped out of bed.  Very quickly she jumped back into bed by me and got as close as possible, laying down differently than she usually does.   I don’t know how to explain it.  I sensed that something was wrong.  I reached my hand down to touch her and she was shaking so much and not purring.   She has not done that before.   Something obviously scared her badly.  I petted her softly for a few minutes and eventually the shaking stopped and she started to purr and we both went back to sleep.