Weird movement

So my boyfriend and I have been active for a couple of months and sex has been good. Some days he wants it and he’ll indicate it, and on days I want it to happen, I hint it towards him, but multiple times in thats day lmao. I’ve noticed an issue recently when we have sex. Whenever I’m either in cowgirl position, or if he’s sitting and I’m on top, there’s a weird movement inside, like as if it’s popping in and out of place. Can someone help me figure out what’s happening?