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.