It might be LEMS. Did they do those tests?

If you do have MG, ditto on what Candi said. You should not be running until it's determined exactly what you have. If it is MG, you could literally go into a crisis while running. If you insist on running, at least carry a sheet of paper that relays this suspicion and says to dial 911, plus have a cell phone with you.

Have you had a brain MRI?

Glucose issues can make someone go "weak in the knees."

When I was younger, I could do more and there would be more times I could sustain an activity. Everyone with MG is different.

Do your legs feel like lead weights? That's classic MG.

Try leaning your arms on a counter for a couple of minutes. Do they start to give out?

Try the upward gaze with your eyes, looking upward without moving your head. Do your eyelids or eyebrows droop afterward? Does it hurt to do it?

If you have a stairway, or steps of any kind, try going up and down two steps repeatedly and see if you get the same funny feeling in your legs. And check your pulse before and after (after resting for a few minutes).

Just be careful, whatever you do.

Annie