Why would RPM stay low and temp guage go up?

Why would RPM stay low and temp guage go up? After driving maybe a mile, my rpms wouldn't go past 1500 trying to go up hill, and my temp guage would go up (almost to the line) when this would happen. Driving flat, or downhill, my rpms would be at what I think is still kind of low at around 2000, and the temp guage was normal.