How can i be in shape but not in shape?

Tealtop1
i'm pretty fit for the most part. I've been working out since I was 13, i have the flat toned abs, toned muscular-ish arms, and i eat pretty healthy as well.

when i workout on my own i'm fine, but at my university i die in the classes.

every time i finish a class (like an ab class or tone up class) my entire body is sore afterwards, it's hard to move, walk, and just sitting up in bed is a struggle. yet other people who take the class (including my slightly overweight friend) are perfectly fine the next day.

so i'm confused. if i workout and i'm in shape, why do the unhealthy people bounce back quicker than i do?
How can i be in shape but not in shape?
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