I'm having issues with my 2012 ford focus heater?

I'm running my heater for over 20 minutes on my way to school and by the end of my drive one of my 4 vents is blowing luke warm air, the other 3 are blowing cold like the A/C. In March or April i had my radiator replaced and my coolant was flushed. I read that the hot coolant is what heats up something to blow the hot air? Also read that when the coolant is flushed it may clog something up which stops it from blowing hot air.

Yes, the hot engine coolant is what heats the interior of the car. Have you check the level of the coolant? If you don't know how, ask at a gas station…

March or April are getting on for 8 months ago. Have you checked your coolant level in that time?

Sounds like blend door actuator