A common problem when work is done on plumbing, like replacing a water heater, is a valve is turned off to make the replacement. When the replacement is complete the valve is turned back on but only partially. This allows the system to fill while allowing a quick shutoff incase there's a leak. Sometimes the workman forgets to completely open the valve prior to departure. If this is the case when you turn on your hot water it will have almost full pressure for a second as the standing pressure is high. When you open the tap the high pressure drops and the partially opened valve won't allow full volume so you have slow flow. Check all the valves.