Well depending whether you are using a mixer or taps will determine what the answer could be. If it is a mixer, the o rings inside could be coming adrift and should be replaced. In a tap, the jumper which holds the washer and is attached to the headpart could be loose and must be replaced as well as the washer.
Seats in the main valve body are leaking by, allowing water to go to the shower head.
full block of main drain. shower is lowest open area for back up to occur.
you have trash in your line take the shower head off and remove trash or possibly in water valve at shower
heat
If your motor is actually starting and not just turning over because the starter is forcing it to then I would check out your ignition switch to make sure your getting power to all the right things in the "Run" position as well as the "start" position.
It would have to be the controls or the shower head. Distance from the heater or size of supply lines shouldn't have much affect. The only variable would be the control or shower head and either one could have a flow restrictor in it or both.
You may need to replace the heater\ac control switch, it may be stuck in the off position which would prevent the blower and the ac compressor from turning on.
Cotton gets wet. By the end of your shower, a cotton curtain would be drenched and would fail to serve its purpose.
i am not a mechanic but i would check to see the wires at the fuel pump have not been reversed, secondly i would check the power wire is not earthing out somewhere along the line because only a dead short can cause a fuse to blow. jacken.
If you were leaning over a lot verse standing straight up in the shower. You may notice that you have acid reflux or coughing from acid irratation when you bend over.
most likely a bearing has gone out
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)