Good Heavens, call your surgeon asap! Do mention position (& length of time upright) is a factor and how long this has been going on as it is very possible you are over draining.

I have never heard of a valve that needs to be switched on before. I'm familiar with fixed, programmable or self-regulating/adjusting valves (e.g. OSV). I would really suspect you have one of those - it must be draining something (or you wouldn't need a shunt) and knowing the setting if it is a programmable would be important.