Every new track I record gets shifted forward just a bit.
 
This never used to happen.
 
What could be the reason? And, how can I put things back to normal?

You have given no information about your system and so how are we supposed to answer? 
 
Version of Sonar
Basic Computer specs and OS
Audio interface and driver mode
 
But my guess is your experiencing sync issue because your not using a ASIO driver. 
Best way to test is to perform a loop back test and see how your overdubs line up to the original. 
Below is an example of the results of  recording the output of Sonar back to the input using a drum audio track made by freezing a synth. You'll see how in some modes the re recorded track is late. Only ASIO is in sync. 
 

 

it is amazing how ambiguous a simple word like forward can be in some contexts. Do you mean it is shifted forward to an earlier time along the real time dimension (starts earlier than expected) or it is shifted forward in the direction of the now time marker movement i. e. to the right on the waveform display (starts later than expected}?