Pressure dropped in all of the apartment

I live on the fourth floor in an apartment building. I bought the apartment 5 months ago. At first the water pressure was amazing, we enjoyed LONG showers because of it. Then something happened, we can't point out what it was but the pressure in all of the apartment, cold and hot, bathrooms and kitchen, dropped and has been low for the last several weeks or month.

Any idea what it could be or where to even start investigating?

Did you try calling the apartment building maintenance or management?

Do you have your own water shut off to the unit?

Do You have your own water heater or is it common to all units?

Have you met your neighbors yet? Ask them if they are having the same problem.

Here an apartment is usually something that you rent and the entire building is managed and owned by others.
If units are owned individually, then we call them condominiums.
We see a lot of apartment building here being converted to condominiums.