How come I shudder after I urinate?

Autonomic response. The act of voiding requires vagal and sympathetic nervous system activity in a coordenated fashion. The bladder muscle is responsive to vagal stimulation and the bladder neck to sympathetic impulses. Shuddering is like an involuntary muscle contraction also related to these neural impulses as a spill over response. It is not found in most people but you have it.