water pressure?

Over the last 9 months the water pressure coming upstairs into the shower, sinks etc has dwindled. The pressure downstairs if fine. I have cleaned aerators and water co. says all is ok from outside to inside of house. This again appears to be true cause downstairs pressure is aok. Something had to have happened for all incoming water sources to have lost pressure, not just one source. Do you have hopefully fix for me.Am now unemployed so plumber is out of ? for now. I am all for longer showers just with someone.Thank You