Why do I start shaking when I work out o_o?

Lmao literally, i'm thin and everything but I want an abdominal line and faint abs. When I do crunches and sit ups, or anything like that my body starts rapidly shaking and it makes it really hard to do it? Its mostly just those two types though.
I can run, dance, bike, and do mostly any other work out like any other person.
Why do I start shaking when I work out o_o?
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