This issue is complicated by the fact that there are multiple definitions for the word "shake":
- (of a structure or area of land) tremble or vibrate. "The ground shakes."
- move (an object) up and down or from side to side with rapid, forceful, jerky movements. "I shake the bottle."
- upset the composure of; shock or astonish. "Seeing the children like that shakes me."
Let's first clarify that the colloquial "I'm shook" is using the third definition. Now, let's look at "shook" vs. "shaken":
Seeing my parents upset shook me.
I'm left shaken by this experience.
In the second example, "shaken" is used as an adjective, as past participle forms of verbs sometimes can be: "My eyes are swollen." In both "swell" and "shake", the past tense ("swelled" and "shook") cannot be used as adjectives ("my swelled eyes" and "I'm shook"), whereas the past participle forms can ("my eyes are swollen" and "I'm shaken").
The reasoning for this distinction (past tense vs. past participle) is that past tense has a fixed time point, whereas past participle does not. As a result, the past participle form describes actions that could have happened at any time, and so can also be used as an adjective -- as things that are red could have become red at any time, things that are swollen could have become swollen at any time.
Yesterday, my eyes swelled up. They are now swollen.
Yesterday, the experience shook me. I am now shaken.