thats a good question. lol
to me, it feels like the beginning of test flu (if you've ever had that). not the sickness, just your body is beat down. I can still get into the gym and train, but my lifts suffer and it starts to effect me mentally.
im sure its different for most, and i rarely ever got it when I was ON, so I'm sure that has alot to do with it, but, and I'm not saying you aren't, but i train my body hard as fuck. I think most guys don't feel it because they really don't train with max intensity. Again, not saying who does or doesn't, but i watch guys train. If you are easily blowing through your sets, even if there tough, to me thats not intense.
When i get done with a set of squats... i have to sit down.
My legs are shivering and I need time before i get down again. If i wasn't afraid of puking... I'd do it nearly every workout. lol
it took me years to learn how to do it, but once i learned what it meant to train with intensity, y whole wolrd changed.
But this is the first time I have trained like this natty, so I'm sure that has alot to do with it. Plus I'm not 25 anymore.