I find it hard to believe that they let you go home and drive such a distance, after PLEX, without checking your blood pressure (which seemed to be on the low side to begin with), and your electrolytes.
Do they give you calcium tablets during the procedure? The citrate (used as an anticoagulant to avoid clotting of your blood inside the tubing and the machine) temporarily depletes your calcium. Some people are more sensitive to this than others, but it can rarely even lead to worsening of MG symptoms.
We give Tums routinely to all our patients.