DrMatrix, are you SURE you fixed it? Might wanna take another look at that baby . . .
Uh… that is even worse… I think.
You really shouldn’t wait to get this checked, I had a co-worker who had an improperly grounded electrical system. He discovered this one morning while washing up at the sink. His house mate discovered it when he found said co-worker dead on the floor. Water heater was feeding 220v into plumbing.
Ok, you’ve convinced me. I’ll get someone to check it out. Thanks for the warnings, guys.
Saltire: house is about 10 years old. Three socket outlets. I’ve heard of ‘ground fault interrupters’ before. Possibly the Australian system is different to your American system.
DrMatrix: why do you hate me so? 
A person where I worked in the Army found out that the Coke machine wasn’t grounded correctly when he took a drink out of the drinking fountain while simultaneously pushing the metal button for a coke.
Lightly zapped lips.
NOTE: ELECTROCUTION FEELS “BUZZY” I’ve zapped myself with 120V several times after thirty years in electronics. It doesn’t “sting,” instead it has a very noticable painful throbbing VIBRATION. If you don’t feel a distinct vibration, then it’s probably not bad grounding.
But better safe than sorry. Yes, I’d have it checked out immediately. There shouldn’t be any AC voltage between sewer pipes and cold water pipes at the tub. If there is, you’re standing in a death trap (wet feet on metal tub, touching the metal shower head with wet hands.)
On the other hand, here’s a possible explanation. Get a piece of steel and a piece of copper, clean them well, then taste each one. No taste, right? OK, now hold them together, then touch your tongue to both of them. YUCK! Tastes like corroded metal. It IS corroded metal. Your wet salty tongue is acting like the electrolyte of a battery, and the copper and steel are acting like the battery plates.
If you stand in an iron bathtub and touch a chrome-plated shower head, again you’re forming a battery. However, your skin isn’t a very good conductor when compared to the salty wet meat inside. If there are HOLES in your skin, the electric current will find a path. This is a feeble direct current, and usually would be too weak for you to detect. But if it’s passing through your open wounds, yowch!
If all the metals were the same (iron tub, unplated iron shower head) then the “battery effect” wouldn’t happen, and you wouldn’t feel any sting.
Narrad, in Australia our electricity is earthed to our cold water tap (usually your yard tap - something I only discovered once when we had powerlines down). Bearing in mind that our power supply is 240 volts, I’d be inclined to get it checked out by an electrician just to be on the safe side.
Do you have earth leakage protection fitted to your powerboard?
I get shocks from shopping trolleys and the like all the time, but what you’re describing doesn’t sound like those kind of static zaps.
I like DrMatrix, but I wouldn’t let him near any of my thread titles. I changed it yet again. For those who might be wondering, Dr.M’s version was “Why does touching the shower hurt head me?”
Oops. Sorry about that.
DrMatrix
Check your hot water heater because I once had a similar experience and it turned out that the element in the heater had gone bad and was shorting out. That was the only time I ever got shocked (slightly) from a shower. Does the water tingle also, because mine did mildly? Either way, it sounds like you have a grounding problem that you need to get fixed for you should not be getting shocked from the shower head so get it checked before it gets worse.
Would putting a voltmeter on the shower head tell us anything? I’m not sure where one would ground to though.
Is it an electric shower - does an electric unit heat or pump the water? have you put nails or screws in the wall without checking for cables recently?
It could be a residual earth problem.
THe only thing that it certain is that an electrician needs to look at it ASAP.
Your house probably has copper pipes throughout so if there is a current flowing, all taps and radiators may also be live!
This actually happened to me but with a bathtub. So, being a guy, I waited a few weeks then
decided to call the gas comp in California…they had a guy out within an hour. They told
me thats because it was so dangerous. He had to reground at the pole, the top of the house
& the electrical box & it worked. Whew. BTW, it cost nothing.
bbeaty: I’m not sure this is a battery thingy in action. My shower has a tiled floor and the drain is plastic. But this description**
**sounds exactly like what’s happening. In my OP, I alluded to an explanation for the stinging that I vaguely remembered. Now I remember that it was on a science radio show; and it was something like the process bbeaty has kindly provided.
Still not sure how electricity is running through my shower head though. Electrician coming soon, hopefully.
Omnivore: the water doesn’t tingle. It only stings when I make contact with the metal head and only when I have open wounds on my fingers.
Pergau: the water heater is both solar- and electricity-powered. I haven’t done any nailing lately.
Umm, I mean I haven’t done any screwing lately.
I mean… ah, forgettit! 
Also, it turned out one of the circuits was doing 122volts. This accounted for the extra
that went through the pipes. But I also noticed a slight flicker on my computer monitor from it.