LOTS of possible causes.
I get this way when I've been in too much pain for too long... it's my body starting to go into shock.
My sis gets this way with migraines (I also get migraines, but I start mixing up words like a dementia/alzheimers patient, and then get my own private black hole).
About 70 different infections come to mind
About 20 different disorders
About 15 diseases
Too many to list Environmental causes (allergy/ mild poisoning)
9 different imbalances
And I'm just a student. Shaky, weak, nauseous probably has 300+ differentials attached to it from ear infections to blood sugar to parasites to neurological. A visit to a doc will rule out half in the first 5 minutes. The rest get tested for in a likiehood schema (just like a baby crying gets wet/hungry checked first, then pain... we don't look for a bug bite first every time our baby cries, we do most probable first, and work our way down the list).