Bucking when lope starts?

The past month or so my gelding has been bucking when we start to lope. It's not huge get off of me bucks but little hops. Or that's what it feels like. Anyways, I think it's just because of it being so nice out. It only happens the first or second time we lope. I'll try loping he'll buck then I circle him and try it again normally by the third time he stops. I'm 98% sure it's not pain and it's just "happy". But the vet is coming out in a couple of weeks to do shots and that stuff. And after I rule out pain how to get over this stage? And it doesn't happen with just me riding he did it with my dad and stepbrother.