I have low hot water pressure. This happened previously, about 1 year ago. The plumber cleaned something from the top of the water heater. Could this be the problem again? How can I check it?

I suspect you have low volume flow which appears to be a pressure issue at your faucets. Some heater have heat traps or nipples on the top and they can slowly become partially closed. Read this, is this what he cleaned out? Energy Savers: Install Heat Traps on a Water Heater Tank for Energy Savings