That could be from a cadence you're lower body is not comfortable with yet. I'm just getting to the point where pedaling at 95-100 doesn't feel like I'm out of control or another way to but it. Feeling I'm behind the rate I'm spinning at. As in, out of time. Pushing a bit late on each stroke. To me it feels like I'm windmilling or bouncing on the seat. I'm a year old rookie so I could be just full of crap.