Most water inlet systems incorporate a non return valve (NRV)or have one installed just after the inple facility to prevent exactly what yours is doing. Without knowing the make and model of the water system we cannot be specific about what has happened, but in general it would seem your NRV has failed. The question is why? The most likely cause (at this time of year) is debris preventing the valve from closing, Perhaps you may have a water filter that you've forgotten to change, as its old it may now be spilling its content (usually black carbon granules)?
The other common failure is after winter if the water system hasn't been properly completely drained, the remaining water can freeze and damage the NRV.
In either case if you can open the NRV without damaging it you may be able to either clean it or reset the 'O' ring, but ultimately you may need to replace the NRV.
