feeling of ball between legs

So I feel like I am sitting on a ball when I sit down and relax my back. Not to feel like that I have to keep my back straight. As soon as I arch my back I feel the ball. I don't know what is it ?? I'm 29+2 days. Also I feel my hips and muscles of my legs are sore. Like I worked out and your muscles become sore. I'm super tired though so maybe it's that. Anyone else feeling like that ??