Water pressure problem - plumber tapped into kitchen hot/cold water line to add bathroom sink on other side of wall
The most common reason this happens is not anything to do with the plumbing work done, but rather when old shutoff valves are closed and reopened the rubber washers crumble and clog the screens in the faucets. Have your contractor take apart the kitchen faucet and check the screens for debris. There is usually ones under the sink before the cartridge depending on the design