I have taken Imuran for a long time and do not have the feeling you describe from that particular medication.
I am also asthmatic and have taken rounds of Prednisone for that even before my MG diagnosis. Steroids can keep you awake for days, especially when you first start them or first increase a dose.
If you feel like your muscles are all twitchy under the skin, that may be related to Mestinon.
Annie is absolutely correct when she suggests you speak with your neurologist about this.

Jenna