After I have sex I shake and feel nauses why does this happen?

Vagus nerve effect. Intercourse can stimulate the part of your nervous system (the vagus nerve) that controls nausea, shivering, sweating, pulse, and blood pressure. You could experience the symptoms you describe, as well as feeling lightheaded and queasy - some women would pass out briefly when the pulse and blood pressure drop enough from this stimulation. These symptoms are usually transient and harmless.