Water pressure in the house

I had my water pressure reducing valve replaced yesterday. It's located about 12" above the main water shut of valve, about 3 feet above the PRV is a water pressure gauge. The PRV is factory set at 50psi.
Everything seemed fine for a few hours, however, the water pressure was at 70 psi when I checked it this morning @8am (after I took my hot shower). Further investigation indicated that the pressure at the thermal tank by the hot water heater was 38 psi (the tank and the hot water heater were replaced in June 2008). I added air into the expansion tank to 58 psi. When I left the house @9 am the water pressure gauge that is above the PRV said 58 psi, same as the amount of pressure in the expansion tank.
8 hours later when I got home from work the water pressure gauge says 70 psi, I checked the pressure at the expansion tank and it says 56 psi.
I can't understand why my water pressure doesn't stay at 58 psi and don't know what caused it to go up to 70 psi. Please help.

If you have a bladder type well tank it sounds like either it lost its pressure(2lbs. less than the pump cut in) or the bladder is ruptured. I just recently had my well tank go bad, ruptured bladder. If you have a tire pressure gauge you can check it. Turn off the water and drain off the water pressure to the well tank and check the pressure at the tire valve on top. If your cut in pressure is say 30psi the tank

If the plumbed in pressure gauge and what you read on the shrader valve of the expansion tank differ, one of the gauges is wrong (could be both!). Unless you shut off the water and release the water pressure by opening a valve, the pressure in the tank will always be the same as the system water pressure (unless the bladder tank is set higher than the water pressure). If the expansion tank pressure is set higher than your normal water pressure, it will only accept water when the pressure rises above the air pressure, which could negate the effect of the pressure regulator if you took it to an extreme.