Why do I feel like I’m suffocating when I lay on my back?

To me this might be silent GERD. It’s like heartburn, but without obvious symptoms. A gastroenterologist can help you.

Here is a good description of causes:
https://my.clevelandclinic.org/health/symptoms/orthopnea
Also, sometimes one needs to change Specialist because Specialists have subspecialties in their specialty. To get the best diagnosis look for someone with depth in a specific condition.

Since all your diagnostic tests were negative, I would tend to agree with otter 2154, silent GERD. Would not hurt to consult a GI physician if not already. In the meantime, perhaps you just need a slight upper body (trunk) elevation in order to breath easier....using pillows may be beneficial. Excess gas can also cause shoulder/neck pain which is called referred pain.....this would also coincide with GERD.

Would GERD limit my breathing all the time even when vertical ? Why does my air flow stop when I lay down?

I hadn’t heard of Orthopnea. Thank you.

If it is GERD, which is what I have, you need to do the following: Raise the head of the bed up. I use pillows under my mattress, one on each side. Don't eat several hours before bed time. Take Pepcid AC ( over the counter) once a day and use Gas X ( one with every meal). Sleep on your left side, not the right. Most of all stay away from spicey acidic foods. Just saw my gastro Dr. this past week. Best of luck

GERD and Asthma are closely related. If GERD is not controlled well, there is usually an increase in asthma symptoms. Pillows or blankets under the top of the mattress will be helpful - and be less painful for your neck than sleeping on more than 2 pillows. Orthopnea is when there is difficulty breathing when lying down (common medical term). Watch spicy foods - and tomatoes which are highly acidic (including tomato sauce). Hope this is helpful.

You can also raise the head of the bed by placing blocks, adjusted to the angle that you want, under the top legs of the bed frame. That way you don't have to worry about pillows slipping around and having to readjust them,

Get in to see a good Pulmonologist and GI MD