Different feelings

I went back to lesson 3 expand to absorb condense to project. While doing the seated meditation I noticed soon in to it that I had a warm sensation in the dantien it seemed to be horizontal circle and not a sphere. That was new. I have felt a kind of fulness there before but not to the point that it felt like an actual shape or that I could feel its specific dimensions. Later, I noticed my mid back becoming either tense or feeling a stretch  or maybe both. It did not feel comfortable so while trying to adjust my posture i noticed that my dantien went a little nuts for about 15 to 20 seconds. Nothing to intense, but it began to...spasm. not painfully. The whole area began to rapidly get a wave like movement to it. I wouldn't call it a vibration, though technically any movement with frequency is a vibration so it was a vibration, because it wasn't buzzing. It was more like the speed of a fast drum roll on a floor tom. If there are any drummers reading you can get a feel for what I felt. Thankfully it was in the dantien and not higher. It remained below the navel sitting in the pelvic bowl. Any insight from Sifu or anyone else is welcome. How can I get such fulness in the dantien when the back doesn't feel like it is releasing correctly?