I think you'll find that there are plenty of people who will have a model/paradigm for why this is and what this is... but ultimately I wouldn't obsess about the metaphysics of it. It might be the unrepression of freudian libidinal energy, the manifestation of jungian shadows, the alignment of the chakras, the opening of channels, insights into the 5 elements, etc. etc.
All of those are going to be generally descriptive and might be useful... but the problem with most of these models is all the baggage that comes with them. If you accept the freudian un-repression idea, then how much more of freud's stuff do you need to read/know/believe? Etc etc. And ultimately it's important to know that these maps are just made up (perhaps useful, but mostly made up). The Buddha was pretty good at pointing out the futility of metaphysics that couldn't be verified/falsified.
That said, I'm going to use this particular map, because it is more practice oriented rather than philosophical...
In general, if you are continuing to consistently practice and your practice is already strong enough to go on a 10 day retreat, then chances are you are working on the different energies associated with the Arising and Passing stage and the dukka nanas. There can be lots of electrical and emotional releases that occur. Basically a lot of subtle stuff seems to bubble up from the depths and needs to be experienced. Sort of feels like a combination of detoxing and discharging electricity at various times.
The map that I'm using here is described in much more detail here:
30. The Progress of Insight – MCTB.org It's one of those maps that is generally true for meditators, but isn't necessarily true for an individual. So definitely disregard if it doesn't seem to apply to you.
Hope this is a little helpful. Definitely keep asking questions, this stuff doesn't seemed to get talked about much as it should. It it is very common with most meditators who have a consistent practice and who go on retreats.
