How long have you had her? How long has the tank been set up.
if she is going to have fry have you provided her with a hiding space and lots of hiding spaces for the fry.
What tank mates does she have?
Often any female going to deliver tries to hide and sometimes will shake. All you can do is watch what happens and if you see any fry catch them and keep them away from the other fish if you can.

 

my older female platy is doing the same thing except she isn't lying down but she chases my male swordtail away