Its probably functioning as normal.
The lower the outdoor temp, the longer run times you will have. Therefore the indoor blower will run what seems like constantly, especially at night time when you are home to observe the system
Occasionally the system will turn on the electric heat strips to help you stay at a desired temperature. That is probably when you feel that the air is dramatically warmer.
If it really concerns you though, just cough up some cash for a service call.
It should not take more than an hour or so to check it out.