What stage is this?

I was doing a session for about an 1.5-2 hours the other day and was using the Helix Syn for around an hour, followed by the Progasm. I noticed an unusual feeling that I don’t normally get in my sessions (which normally feel good but are mostly uneventful).

I felt a sudden vibration, and a clenching that would come on and off in my abdomen. Occasionally this would coincide with the flexing of my PC muscle that I was not doing in a voluntary way.

I did not feel a release or a sudden feeling of pleasure like a normal orgasm. I just had this vibration and clenching going on, which felt good but not like a full regular orgasm.

I have been at this for a few years and never got that far before (I was using a do nothing approach).

Is this a prostate orgasm? Or what stage of the process did I get to? I am a little confused. It went on for 5 minutes or so and then I lost it and ended the session.

Any ideas?