Why am I running out of hot water?

I took a short shower. I noticed the hot water steadily cool down until it is just lukewarm. I'm the only person living at my apartment. How is it possible that I already used up all the hot water? Could my hot water heater hot be filling itself up with water and I just used the last bit of it? I hear a hissing sound coming from the heater. I don't know how to check if there is any water in the heater. I notice there are two water pipes connected to the heater, one is cold and one is warm. So I'm sure the heater is still doing heating but I'm not sure if water is being refilled. What do I do?