Check the date on your water heater. May be the first 4 digits of serial. If you can't figure it out, post the brand and serial.
If your WH dates from roughly the mid 90's, it was part of the dreaded "dip tube problem" , which causes the symptoms you have. The dip tube can be replaced, but if the WH is that old, you might rethink spending money on it.
Other possibilities: do you have a recirculating pump on it? Is your gas or electric bill up, possibly indicating a leak or crossover in hot water?