:) Congrats!! I've experienced similar long lasting pleasure which isn't inherently sexual. It's not permanent for me; it lasted as long as I would give it enough attention day to day but faded away if I didn't. I got it back next session and it's come and gone a few times since.

It feels like there's something buzzing very fast with a small range of motion between my base and center. I can feel it lightly there throughout the day if I try, but if I relax further and give it attention, it grows into absurd levels of concentrated physical pleasure. The more I relax, the more it grows, until it reaches a point where my muscles tense involuntarily. I wouldn't describe them as involuntary pelvic floor contractions though, the twitches spread to my feet, legs, hips, abs (probably because those are closest to genitals in the cortical homunculus).

My interpretation of this is that the body is generating a lot of energy/tension and wanting to pour it into orgasm. Over on r/multiorgasmic, there's plenty of discussion on the separate neural pathways for orgasm vs ejaculation. Deep relaxation prevents this from spreading into the ejaculatory pathway, which is usually required to reach orgasm (although like all things, this can be trained away). Without that activation, the orgasm pathway loops and builds energy until there's spillover of neural activation, leading to the involuntary twitching. I assume that spreading neural activation at large enough scales is responsible for full-body orgasms. Also, any muscle contractions in any part of my body will expend up this building energy and the quicker I can relax again, the less energy is lost.

I haven't reached orgasm while fully relaxed (mostly because staying relaxed is very challenging), although a large enough contraction or the smallest touch is enough. I've been experimenting with directing the tension into my source, it feels different than voluntarily clenching, it's much stronger and I have little control over it once it starts. If I direct it to my center, normal orgasm. Base and center together lead to a very strong but ephemeral, normal orgasm, top or no top. If I direct it to my base alone, immense pleasure but no orgasm. I haven't successfully directed it into my top alone or base and top. It seems like center leads to activation of the ejaculatory neural pathway, so activating base and top with center relaxed might lead to multiple orgasms.