Hard Cough

I am on antibiotics for pharyngitis and on a mycolytic for the phlegm that is too thick and too much. The medication has worked so far and especially the mucus and phlegm are dissolved, more light colored and cough-able. Now my question is , why when i stand up, i get to cough lots out and be relieved for quite some time, and while i am in my bed (as prescribed) i don’t get to cough out much. And also, logically, should i be standing up more often?
Thank you.

When you are lying down your abdominal contents are closer to you diahragm, the liver, stomach, some intestines etc. When you take a deep breath to cough the excursion of the diaphragm is restricted to a degree. This has the effect of lowering how much air you can breath in and therefore out. When you cough there is less air behind the mucous to push it out.
On the other hand, while standing, there is no restriction placed on the diaphragm and you can get more air in.
Most people the reverse is true because gravity assists in the drainage of the mucous while lying down. Your congestion is higher up. Sitting up on the side of the bed may be enough. Or standing with assistance(I don’t know your situation) when you need to cough.