I definitely also think moving around is the way to go, but in regards to feeling uncomfortable when sitting straight, it helped me to do stretches.

I’m guessing, if you’re always kind of ‘curled’ up, the sinews and muscles and such at the front of your body contract. So, if you then go into the straight position once in a blue moon, everything at the front of your body has to be stretched further than it usually would be, which is uncomfortable.

Well, and if you give it the ol’ morning stretch, those sinews and such will be stretched beyond the normal straight sitting position, so that the normal straight sitting position is within the comfortable range…