Why,in the past week, have I lost significant water peressure in the tub/shower fixture?

do you know what kind of piping you have. Is it galvanized? if it is galvanized sounds like it is time to replace your water piping. If it is not that what kind of shower valve do you have? is it a single handle or numerous handles. If it is a single handle you can most likely just rebuild the valve and your problem is solved

Without question you have deposits restricting the water flow in your shower head. Buy some CLR at Walmart or your local grocery store in the cleaning section. You can either remove the shower head (it screws off counter clockwise) or fill a container with the CLR and hold it so the shower head is submerged in the liquid for 5 minutes. This will clean the showerhead.

As for the washing machine, turn off the supply valves to the machine, and remove the hoses from the back of the washer. There should be a little screen inside either the hose or the male connection on the machine. Chances are it is plugged. Remove the screen and clean it, and then replace your hoses making sure you hook them up correctly.

Also, it may be the "Dip Tube" going bad in the WaterHeater. Lil chunks of plastic will clog the strainers in the showerhead/washer hose/refridge icemaker line etc...

If you have noticed lil (probably white) bits in your sink after using the hot water than its prolly your problem.

Possible lime/calcium build up...buy a lime/calcium/rust remover (Walmart has) and soak shower head in it for directed time...if not try something else, this wkd 4 me

i would check to see if anything is linking