Heater??

It is not the heater, but the balance of pressure in the plumbing. Let's say you are using a 50 - 50 mix of hot and cold to get the shower temperature right. Someone turns on the water in another sink. The pressure in the system drops and they are now using some of the hot water you had so the mix drops to 30 - 70 and you get a chill.

There are temperature sensing shower valves that will eliminate this. I have an anti-scald valve that will not let the temperature get any hotter if say, a toilet is flushed with cold water, but will still allow the water to run colder if the hot is turned on elsewhere.

If you are using a standard tank type water heater, it is not the heater. (Tankless units are different).

Usually it is because you are mixing hot and cold water. When someone turns on the hot somewhere else, the pressure for the hot goes down and you get less hot water, but the same amount of cold. The problem is complicated by old pipes with build up inside restricting flow, pipes too small or an inadequate supply.

Most newer shower valves have some sort of device to reduce the effect. Even changing the kitchen faucet to water saving variety may help.

If you have old plumbing, you may want to consider updating it. The old stuff may well be close to the end of it's useful life and replacing it now could save you a lot of damage later.

Thanks guys for answers.Can you please tell me where is shower valve located.I'll try to replace it thanks

First place i'd look- the basement. You'd be surprised how often you can find an old shower valve in the basement. But if you wanted to find the one that works your shower, i'd look - (wait for it)

-in the shower.

But you don't need to replace valves, you just need to shower when no one else is using the water.