When you say stretching, can you describe it more? Does it create a bulge under your chin, or are you feeling more of a tautness?

It's more of a tautness. Like the lower jaw is moving forward. It's not too pronounced, but there's definitely a stretch happening. 

The tongue is connected to the hyoid bone, which is also connected to the mandible and to the skull (connected by muscles, hanging to be able to move freely while speaking, swallowing, yawning, etc.). So when you place your tongue on the palate the hyoid bone is raised too.

I experience tautness as well. In my own case, I think it has to do with my muscles finally being used in the way that they should be and experiencing new sensations. Over time, I've felt the intensity of that tautness diminish, it now just feels normal.

It's always a little tough to be practicing oral posture on my own. I'm never sure if I'm doing it correctly. I found confirming or getting adjustments from a myofunctional therapist really helpful. Things I questioned for weeks or months, my therapist was able to answer in seconds.

All this is to say if it feels right, and you're gaining positive results, chances are it's a good thing. But there's no way of knowing until seeing where things are some time from now.