If your water heater is an electric water heater it could be that one of your heating elements on the water heater could be out and not heating the water up enough so it causes the hot water to run out faster.
Another causes could be that the water heater has built up a lot of sediment in the water heater tank and not getting full enough with water to heat up.
So your water heater could be partially full with sediment from the water which takes up more room in the water heater tank so you don't get as much hot water.
I've once seen a water heater halfway full of sediment which made it lose it's capacity overtime and it was 25 years old and ready to be replaced anyway.
But if you clean the sediment out by flushing the water heater tank out often enough the sediment won't build up as much.
I would check for sediment or a broken heating element if the water heater is an electric water heater but the three most common culprits of a water heater running out of hot water so fast suddenly are sediment build up, a faulty heating element and a broken dip tube.