If you lie on your back with bump squishing you down, you end up cutting off your blood supply from the aorta (your main artery). The best position (although I can't stick with it for long) is to lie on your left hand side with a pillow supporting bump.
The reason I'm saying this is the jittery feeling is perhaps down to low blood circulation.
If this isn't the cause, it may be something like low blood sugar, and you'll maybe need to start having a midnight snack!! xx