Bubbly feeling by tailbone/butt?

For the last 2 days I’ve had this really weird bubbly/maybe muscle spasm feeling occur for about 5 seconds but I can’t pinpoint exactly what it is. It’s around my tailbone/butt area but not my butthole or the surface. Sorry, tmi. I’m 33 weeks pregnant and I have had gas before but never this bubbly/flutter sensation.

It’s not painful and it doesn’t itch - it’s just annoying.

Any thoughts on what it could be? Maybe the baby? Trapped gas?