It is normal for building water pressure to vary between 20 40 psi or 30 50 psi depending on how your pump pressure control switch is set.
Well pump water pressure varies.
If the overall water level drops that will reduce your storage above the pump and often also reduce your recharge rate.
This can be compounded if the overall water level in the well varies since the water in the well above the pump serves as a store of water that can be pumped out without needing to be immediately replenished.
However if you live in the countryside and have a well water supply your water pressure is likely controlled by your well pump pressure switch.
Unless you have a well and pump your household water pressure should remain constant but it may vary considerably when appliances showers sprinklers or other water using devices are engaged.
If you think your hot water piping is clogged see clogged supply pipes hot water.
As pipes leak or clogs move around inside the pipes water pressure can vary and fluctuate.