Not having had that moment in your body I’d really just be guessing - but if you think of chakras as energy centers, and you think of energy as being very alive and active, it makes sense that upon activation you might experience a physical sensation like that.
Personally I’ve felt pressure sensations, pulsing, stretching in a way, just as I’ve experienced with activating muscles. So when you open a chakra, like you would activate a muscle, I think feeling a physical reaction like that is only natural.
I’m glad you had an inspiring experience, wishing you light on your path as you keep exploring!
Anna