I'm sorry to say that the odds of occular myasthenia becoming generalized are pretty high. Here's a study that says 50% within two years:
http://www.ncbi.nlm.nih.gov/pubmed/12580710

Here's a link that says 50-60%
http://emedicine.medscape.com/article/1216417-overview

The episode you describe, where your legs just can't hold you up any more and you have to sit on the ground for a while, is very familiar to me. I haven't ended up on the floor in quite some time, though, because I've learned to notice the warning signs in time. I also try to avoid being out and about at the time of day (late afternoon and evening, for me) it's most likely to strike.

It's very common, as you know, for an infection to trigger a worsening of MG. As to your question (if I understand right) of whether this could just be a passing episode, I haven't heard any stories like that, but you never know. MG is pretty unpredictable. Rest, rest, rest, OK? And keep that service dog ready to go!

Abby