Our upstairs shower has always had great water pressure... until a couple days ago. It is roughly half what it was. I've noticed no difference in pressure anywhere else in the house. It mysteriously just lost pressure seemingly overnight. The shower in question has a faucet for a tub which does not function (to run a bath the water must come through the shower head) which is inconvenient but in the two years I've owned the house it has not been an issue with the shower pressure. The showerhead is clean. I am not a plumber and this has me stumped. Any suggestions would be greatly appreciated.