If I continue, then I feel uncomfortable after 20 minutes.
You are continuing to interfere with your breathing by “doing” all the things that you describe.
If it feels uncomfortable, then be aware of it but do nothing. If the breath is shorter while sitting, then be aware of it and do nothing. If the back is bent, do not try to straighten it out, just be aware of it and it will naturally straighten itself over time.
You are trying too hard. That is why I prefer to teach my students to recite the Buddha’s name and ignore their breathing. Watching your breath tends to bring out the “control freak” in all of you.
If it feels uncomfortable, then bear it until it stops feeling uncomfortable. It’s just like going to the gym, at first it hurts when lifting weights, but it is necessary to build up muscle mass.
I do not mind your detailed questions. It will be much clearer when you come to our temple. Also, it will be much easier to make progress. To prepare for the upcoming Chan Qi, you should concentrate on sitting for two hours.
All of you please note: The Chan Qi is supervised Chan training as per each person’s individual level. Each of you should train according to your own preferences, abilities and circumstances.