I turned on the heater as it was a chilly 37 degrees

Thanks for taking my question, whatever advise you can give me would be sincerely appreciated. So, I went to go to work this morning and I turned on the heater as it was a chilly 37 degrees. I headed out thinking the truck will warm up and so will the heater. It takes me roughly 20 minutes to get to work and I've got the heater on full blast the entire time. After about 15 minutes of driving like this, I suddenly get a thermostat warning on my dashboard so I immediately turn the heater off as it wasn't blowing any heat anyway and within seconds of turning off the heat, the thermostat drops down back to normal..?! I get to work and park in my usual spot and I hear a fan coming from the engine itself, like a secondary fan and it's pretty loud. After a few minutes of being parked (but the engine on) that fan turns off and everything is running normally. Still no heat though. With the scenario above, what do you think I should do?

Thanks for your time. Charlie