Gt speed wobbles?

So I just bought my first Onewheel, I got a one wheel gt 2 days ago and have 20 miles on it. I have the stock slick tire and I feel like when I start picking up speed the back end feels like I’m getting speed wobbles. My tire is set to 18psi what am I doing wrong? I have my knees bent slightly and have my shoulders square to the board. I used to skateboard and snowboard so I’m not new to board sports.