Hot water poblem. Please help?

Recently I have noticed that I am no linger getting enough hot water ti fill a bath. The pressure from the hot tap gets lower and lower until no water comes out. I am sure that it used to run cold when out of hot water but now the water from the tap just stops when out of hot water. I no longer get the gushing sound of the tank refilling when running the hot tap, it sounds more like a trickle.