The only way to know for sure if breathing problems are caused by respiratory muscle weakness is to do appropriate respiratory tests.

When a person experiences breathing difficulties it always leads to some degree of anxiety. Our body senses very quickly that there is something wrong with our ability to get rid of CO2 and/or get enough oxygen.

A patient with a pulmonary embolism can look like he/she is having an anxiety attack if you do not suspect it. The same is true for asthma, heart failure or any other problem leading to breathing impairment.

But, certain signs and results of tests direct you to the correct diagnosis. Many times reassurance and a calm environment can lead to significant improvement even if there is a significant medical cause, so this too can be miss-leading if you rely on that.

Patients with severe respiratory muscle weakness may appear disproportionally calm because they are unable to show their distress.
This can be very miss-leading for those who are inexperienced.

So, if you are experiencing even relatively mild breathing difficulties it is important that you consult a pulmonologist (with some understanding in neuromuscular diseases) and have a proper evaluation.
If all your tests are normal (including respiratory muscle strength and endurance) it is not likely that your breathing difficulties are due to MG.
If they show some abnormality it may help in leading in the right direction.

They can also do those tests before and after you take mestinon.