I have an issue with my heat and air

I have an issue with my heat and air. They both do what they should do, heat up really hot and cool down really cold, the problem is the air just barely blows out. One day, when I got into the vehicle and turned the air on full speed (4) it felt like it was blowing on the first speed. Still cold, just not blowing hard! The heat does the same thing and the knob works at all speeds, intervals just like you would expect, except at a much lower rate. I have changed the resistor and that did nothing. A friend (not a mechanic) told me to check the relay but I have no clue where that is. Someone else told me to flush my coolant. Sounds far fetched, so I'm coming to you to help me clarify!