Anyone have ideas about what's happening?

At the beginning of each meditation session I experience extremely pleasurable sensations, feel energy moving in a circular pattern through my body, sometimes feel/see white light or coloured light coursing through me. It makes me laugh and smile. Sometimes I want to make vibratory sounds of changing pitch (so I do). My heart feels open and expanded. Sometimes my body feels much bigger than my physical body, or I feel like I'm floating. I can call up this experience pretty much any time - particularly when walking, riding a bike, feeling the wind on my skin, or seeing/smelling beautiful flowers.

After meditating for a while these sensations fade to a calmer state. Sometimes I have mild physical discomfort, but the overall feeling is gentle and stable.

My guess is that I begin meditation in the Arising and Passing Away. But I've read that starting in A&P happens after stream entry. I recently had a cessation experience during an ayahuasca ceremony, but beginning each meditation session with these intensely pleasurable, blissful experiences had been happening for a while before the cessation experience.

I'd appreciate any thoughts anyone wants to share with me.

Sounds like a great start to your meditation sessions. Whether it's A&P or not doesn't matter too much, because the next step is the same in any case. Even if it is, that doesn't imply that you've reached SE. There's a lot of individual variation in the stages.

Thanks, fivebells. Enjoying what's happening and continuing to sit is definitely good advice. I've also been reading the chapter on raptures in Jack Kornfield's A Path with Heart, and it's clearing some things up too.