What causes us to sometimes wake up shaking after we have been asleep?

Vague: Hard to say. If this happens quite often, i would keep some type of journal and write down what you might have done the night/day before- nutrition, activities, meds, supplements, etc. Then get to a dr. And share your history.

Many options: Hypo and hyperglycemia, anxiety, dehydration, certain medications. Regardless the cause the visit to your PCP is warranted for physical, labs and treatment.