I find myself shifting up when above 3000rpms by default, much higher when accelerating aggressively such as on short freeway ramps. This bike likes to rev - don't be shy.Upshifting at 2500 could be the problem. The engine is nearly lugging at 2000. You want to keep it over 3000 pretty much always, which means shifting around 3500 minimum.
Others make good suggestions here.
If the problem continues - check your clutch cable free play when the bike is warm. If the gap is growing too much, you may have a defective clutch cable. This happened to me: my clutch free play when cold was adjusted to spec. When warm, it grew to over 1/4 of an inch. Replaced the clutch cable, problem solved.