Really strange plumbing issue...?

if your shower has only one handle to adjust the hot and cold water, then your shower facet could have a piece of trash in it or it might need to be replaced, I would check that before doing any expensive repairs

WOW, the place to start: first question all of us answering this might ought to comprehend is do you have a septic tank contained in the back backyard or is there a municipal sewer equipment. Being 5' decrease than grade is between the subject concerns you have and that i'm guessing you have sewers because of the fact a septic tank might must be buried too a the thank you to have this setup paintings or have a pit and ejector pump to advance it into the tank. So whats probable taking place is the pipe maximum excellent out of the domicile on the backside element would possibly no longer have lots pitch, or trees/timber have chanced on their way in with the aid of joints contained in the pipe and their roots have created the equivalent of a sieve this is catching the cleansing soap and that's the reason you need to end and permit the water to empty earlier polishing off your wash cycles. instead of the pipe being 4" diameter this is decreased so small that the water is caught and can't drain many times. i've got faith you will ought to have the roto rooter guy snake it out back, yet this time spend a dash greater and get a company that has a video service that can insert a small digital camera on a inflexible line and easily look contained in the pipe to identification the subject no longer purely quickly chop it away. however if this is roots or another obstruction like a overwhelmed pipe you need to get it fastened so which you do no longer run into the subject back very quickly and spend much extra money.

Your shower head has a 'hot' AND a 'cold' water supply. Sounds like a 'hot' water supply issue.

Is the 'hot' shut-off valve going to the shower head turned off? Is there a blockage and/or leak in the hot water supply after the kitchen/bath sink but before the shower head?

Rots of Ruck.

given the additional details, that's really weird. Are you sure the sinks get their hot water off the same heater - no demand heaters under the sinks or anything like that?

Is there a tempering valve that's just for the shower?

check your shut off valves going to your shower. If they are open then you could have a bad mixing valve on your shower. That usually costs $100-200 for the parts and about the same for labor

How far is the shower from the waterheater?? The further the longer unless you have a recirculation system.

If you did not close any tap in that section must be your valve and you should replace it.