Abby, I wasn't able to tolerate IVIG -- but, EVERY illness -- viral or bacterial -- sends me into an MG exacerbation -- usually 3 months long. I'm hoping that WITH you being on Imuran and IVIG that YOUR exacerbation won't last as long.
When we get sick, our bodies produce antibodies -- ALL kinds of antibodies, including MG -- we are non-discriminatory.

So, the good-ole MG "factory" is in full-production mode due to your cold. You are just gonna have to wait until your factory decides to "lay off" workers.
And, you are right -- my MG is worse about ONE day BEFORE I know I am sick with a cold, the flu, a stomach virus, or other illness. The antibodies are in production, I think, BEFORE the viral/bacterial symptoms appear. And, then, I will feel better, MG-wise, for a couple of days AFTER I get over my illness -- and then, take a SHARP plunge as the antibodies seem to go into overdrive. It is a STRANGE disorder to live with. I get sick a couple of times each year -- even though I carry anti-bacterial wipes EVERYWHERE I go and wash my hands like an obsessive compulsive.