Can someone have PEM and no CFS?

What is even the difference between the two? Is it a different name for the same thing? Because if i had to put mine into words it would be more like PEM. I am not fatigued on a daily baisis, i can go to uni, go out, do whatever, and i dont feel much more different than usual. BUT if i go to the gym and start running for 10 seconds my muscles give out and i start feeling other symptoms of LC. Does anyone have any idea why? And i am also scared that i am fighting the wrong battle, what if this all isnt LC? I did do a lot of tests, blood tests a million times, scans of all parts of the body, but i just dont know, this illness is making me crazy :/