This is a manual HVAC setup no automatic temp settings. Prior to this if you put the fan speed on high and put it on the coldest setting the fan speed would decrease. I simply put it to the second last notch on cold and it would work just fine. Now all of a sudden this has switched.. cold air works very well.. while if I put it on hot air the air seems restricted and doesnt blow much air (still has heat though). I don't see how this can be a cabin air filter problem as cold air blows air just fine. Also switching it to internal air circulation doesn't make a difference.
Were you using your phone by any chance? Is so it lowers the fan speed to cut back on the noise as high speed is pretty noisy.