Excess phlegm

I was recently diagnosed with ocular MG. I was started on 30 mg of mestinon 3x day. Then upped in a months time to 60mg 3x day. Started noticing excess phlegm and went to my GP who treated me for bronchitis . Gave me prednisone and the phlegm disappeared.
After a visit with my neuromuscular doctor she upped my mestinon to 60mg 4x a day. More phlegm & coughing. Now I’m being reduced to the 60 3x a day and if I’m still coughing and have this excess phlegm she’s lowering it back to the original 30 mg maybe 4x a day.
Has anyone experienced anything like this with mestinon causing all this excess phlegm & severe coughing ? Thanks for your help.