Weird energy shift after yoga

[Serenitycouragewisdom]

I recently attended a yoga day at a local studio, and after the first session during the relaxation part I felt a physical shift of energy inside my body twice, it felt like a big thud and as if my body was putting itself back in the right place, and then that evening I stirred a little from my sleep and had a kind of vision/feeling of dark black energy leaving my body, has anybody had a similar experience? Or know what this could have been?

Thanks