Hi,

I experienced similar shakes during my first practice session. These were involuntary abdominal tremors that I've had previously and think are due to muscle tension releasing. When I started meditation I had similar tremors in my neck! Perhaps they were a bit lower this time as the focus was a bit lower.

I look at it as a good sign that something is going on. We'd all be much more frustrated and concerned if nothing was happening!

Good luck and keep at it!