Open floodgates

First off, I would just like to say I love this technique, even though I have only been using it for about a week. I do have a couple of questions though.

At the end of the first week I began experiencing strange effects, such as a kind of rocking head movement and other weird sensations in my body which were so intense that I couldn't focus on anything but the movements.

Then, a few nights ago I had a a pretty mellow meditation session at 7 pm (I didn't have any of the intense head rocking and body twitching i had been experiencing before). But later, when I went to bed at about midnight my body began to start twitching -- first it started in my feet as small pulses and then it became so intense that my feet were kind of kicking..this feeling then moved up my legs and into my back where there was this kind of really enjoyable tingling. It was extremely intense, like literally my body was shaking and I couldn't seem to calm down (I wasn't thinking the syllable either)....naturally this also prevented me from getting to sleep.

So my first question is, is this at all common to have the stress relief effects when not meditating? Also, I'll add that it seemed to be related mostly to having my eyes closed. Is it possible I am not resting long enough after meditation?

My other question is: Is there any way to know whether you are focusing on the syllable too explicitly, or a way to self check and make sure you are meditating correctly? Is it better to err on the side of not focusing enough?

Thanks!