Energy All In Your Stomach During A Session

I started to get something a bit different..

In the past, I've had a bunch of leg twitches, but it never seemed to come with any feelings of pleasure or overwhelming sensation.  I've had some one and done good sensations from session to session, but nothing that really stood out.

My last session was different.

Everything seemed to be centered into my stomach.  I got a sensation I can only describe as my entire set of ab muscles squeezing down, as if I was doing a really strainful workout.  This followed by an almost pump of my abs, and then some uncontrollable body movement like you see in those Youtube videos (just not as intense).  It seemed to last about 30 seconds.  The more interesting thing was... it didn't matter if I switched positions, or stopped doing the breathing, etc.  I was able to almost at will make it happen again every minute or two.

But here is the kicker..  There was no pleasure.  It was energetic enough that it made me have to catch a breath.  That was it. 

So, how do I get the pleasure?  Am I supposed to somehow relax my abs?  Is it just a mental thing and I don't recognize the pleasure?