The drop in water pressure can occur for several reasons.
One may be that the municipal water company may have temporarily lowered water pressure in their system to make repairs.
A second, more likely reason is grit coming through the pipes from the water company from repairing their pipes. A lot of grit from mineral deposits break loose from the inside surface of water mains when the water company repairs pipe breaks. These gritty mineral particles build up in your valves and block the water flow. It’s like running sand through the pipes, and the sand blocks up water flow which seems like low water pressure.
Your water pressure to the house may be good but that valve, faucet or pipe may need cleaning. The faucet has to be dismantled, sometimes by a plumber, to flush out the grit and allow the water to flow freely again. This block can occur at shutoff valves in the basement for that bathroom, shutoff stops under each sink on the wall, and in each faucet itself.
Another possibility for dropped water pressure is that your pressure regulator (usually in the basement where the water pipe comes into the house) may be blocked or “worn out” too. The pressure regulator’s purpose is to limit the water pressure so that electronic water valves on your appliances don’t wear out too soon from high water pressure. However most pressure regulators allow too much pressure when they “wear out” but they are also prone to the gritty blockage like the faucets.
You describe typical and usually recurring problems, meaning you may have to “clean” the faucets every few years if you are in an older area with water main breaks. Multiple possible solutions.
First, clean the faucet closest to the incoming water pipe in the house. If you get good pressure at the closest faucet then the pressure to the house is good and each faucet is likely blocked by grit & sand. If the pressure is low after cleaning the faucet (inside or outside faucet), then the house likely has low water pressure.
Typically the low pressure is noticed at the bath tub due to waiting time. Water pressure in your house should be between 40 and 55 psi pressure, ideally 50 psi. (psi is pounds per square inch). Water pressure higher than 55 psi wears out appliance solenoid valves very quickly.