Doesn't take that much to change the velocity 30fps. I've had it come close to that in MD. Between swollen paint and CO2 temp changes on rental guns, we can set them to 250fps in the morning and they will be up to around 290 by the afternoon.
Regs could cause problems if they aren't clean. And it could even be the temperature affecting your lube. Or maybe a combination of them all. Just chrono your gun a couple times through the day.