Does the body release any chemicals, adrenaline, etc. Right before a BM? Always feel shaky/strange right before a BM, then relief after.

Yes. The perception that its time to have a bowel movement is a stressing factor that requires Cortisol.
Having bowel movement itself reuires quite a lot of it.
Afterwards ,when have to shift back into place, cortisol is required too
So my theoretical answer is: its the increased DEMAND ON YOUR ADRENALS ( AND THEIR INSUFFICIENCY TO PROVIDE) That Causes the jittery.