Mr Quasimodo:
I was meditating this morning, and after the "usual" process of gently disengaging from emerging thoughts and drifting to a state of no-thought, I found myself experiencing subtle waves of warmth through my body. This is not uncommon when I meditate and I allow it to happen. It is pleasant and a temporary stage of my meditation. But whereas previously the sensation has been brief and mild, this time the waves grew in intensity and my breathing picked up from the highly subdued level it normally reaches while meditating. It built steadily and quickly to the point where my breathing was very heavy; I was gasping, rolling in my chair, and perceived intense waves of warmth moving upwards in my body. I felt something resembling surprise and simply allowed it to occur and overwhelm me.
This just sounds like pretty straightforwardly moving from 1st to 2nd jhana. You say you're unfamiliar with the canon of meditation, so that may not mean much. Suffice to say that jhanas are stages that the Buddha described that a meditation can often go through in a recognisable sequence. The falling away of thought and the arising of "piti", usually translated as "rapture", are the signs of moving from 1st to 2nd jhana. Piti is pleasurable at first but can be anything from just enjoyable to overwhelming and eventually unpleasant, and this can vary from sit to sit.
When I started meditating it was with TM and I used to get very concentrated very quickly and experience intense, sometimes overwhelming, waves of bodily pleasure which were indistinguishable from those experienced during sex, but with no sexual arousal. I found over time that using a wider focus of concentration (the breath works for me) made the piti less overwhelming and allowed me to go further into the jhanas. The main advice is to stick with your meditation object and let the rapture come and go as it pleases, and this will let you go further. It sounds as if this is what you were doing anyway before you were interrupted. Using a less narrow focus of concentration than your TM mantra, like the breath in the body, might also help.
Thanissaro has recently released a new guide to breath meditation which discusses the jhanas which you may find helpful. It's
here.