My FWB gives me that goin downhill on a rollercoaster feeling in my stomach!!!?

It only last a second when it happens and it's only when I'm talking to him and I thnk about us doing sexual things, what does this mean? Does anyone else know the feeling I'm talking about, I've never felt that in my stomach when is guys I like when it's guys I like it's usually that butterflys in yor stomach going uphill on the rollercoaster