Your main sewer line may be clogged. If you have a clean-out for it, you can unscrew that and see if there's standing water in it. If there is, your line is clogged (or possibly collapsed) and you need to get a plumber to clear it out or fix it.
It depends on your pluming and how the pipes are place
House trap /storm traps need cleaning
If you have a new pressurised water system there is a possibility that the added water pressure could cause a leak.
Bad shower valve.
There are a few advantages of a handheld shower head. It allows water to clean at different angles then a stationary show head which can help with hygiene.
The pedal shower or the Velo-douche was invented in 1897 in England. The device was a stationary bicycle that had a tube that pumped water on to the rider. There is no public record of who actually invented the pedal shower however.
A dirty or clogged screen can cause the low water presser remove shower head and clean. If not it could be calcium deposits built up on shower head if so soak shower head in a cleaning solution for several hours if that dose not work buy a new shower head. (note some shower heads have a low flow re-stricter in some cases you can remove this and get more water pressure)
A malfunctioning water heater can cause hot and cold water to surge through the shower heat. A faulty water pump can also cause this problem.
Its a loss in water pressure to the shower.
you have trash in your line take the shower head off and remove trash or possibly in water valve at shower
It will not work to smoke while in shower as the water will turn off the cigarette or cigar.
Could be water-heater problems or faulty valve.
Seats in the main valve body are leaking by, allowing water to go to the shower head.
the tap assebly could be cross connected if older shower set