You shouldn't be able to feel the Tampon when sitting down and Tampon Should not be uncomfortable when it's inserted correctly.
If you're feeling the Tampon when sitting down it's probably because you inserted the Tampon at the wrong angle and should remove it and reinsert it.
Tampons when inserted correctly should be hard to feel when sitting down or doing anything and if you're feeling pain for too long or it's uncomfortable then it's probably because it was not inserted into your Vagina Correctly.
I've worn Tampons before when sitting down and never felt the Tampon so if you're feeling the Tampon sitting down you should check to make sure it's inserted correctly.