Water temperatures that fall quickly could be caused by a couple of different issues. The first is that your dip tube may have fallen off inside the tank. The dip tube forces cold water coming into the tank to the bottom so that it can be heated. This allows hot water to remain at the top of the tank near the hot water outlet where it can go out to your faucet.
If you have an electric water heater, the problem may be a malfunctioning heating element or thermostat. If the heating element in the bottom of the tank is not working properly, only the top part of the water will get warm. A qualified water heater repair technician can inspect and replace your thermostat or heating element.
You should also make sure that you do not have a leaking toilet. You should check that water is not flowing into the overflow pipe. The overflow pipe is located in the middle of your toilet tank and is connected to some small tubing. If you notice water going into the overflow pipe, you will need to adjust the fill valve so that the water stops at the water line mark stamped on the side of your tank or approximately one inch below the top of the toilet’s overflow tube. You can also use food coloring to check for leaks in the flush valve mechanism. Place a few drops of food coloring in your toilet tank. Check the color of the water in the bowl in approximately 15 minutes. If it has changed colors, you have a leak and should replace the ball or flapper.