I have similar experiences, where my body goes limp suddenly, usually I have 30 seconds to a minute to get to a couch or bed and if I don't make it I end up on the floor. I think it has something to do with an antibody surge like you said because I also feel tingly under my skin during these times, a spreading senstion that moves outward ending in my hands and feet.

If I am feeling generally okay, I am like you and recover in about 30 minutes. If I am experiencing lots of MG symptoms previously, I require a long nap to recover, about 2 to 3 hours.

Almost always this happens after some triggering event. Usually it is stress (99% of the time), but occasionally it is something I eat or smell. It is almost like an allergic reaction and I am allergic to stress. Sometimes a phone ringing triggers this collapse, because the phone startles me.

I've also experienced this by smelling and/or eating cinnamon, which I am extremely allergic to, but in these cases the reaction is a little slower, it takes 5 or 10 minutes before complete collapse.

It is not a fun thing, I've spent way too much time laying on the floor waiting for the weakness to pass.