Why might my car be running hot?

I’m not very car savvy, so I’m sorry if this is a stupid question. Just answer it if you know. :)

Ok, so I drive a Pontiac Grand Prix and my temperature gauge is like this: http://tinypic.com/r/vnhwt1/7

Normally, and for the longest time, the car stayed on 160 pretty much the whole time. Recently it’s been running at 210 but didn’t go any higher. I was told not to worry until it got into the red. But today and yesterday it’s been running between the 210 and the numberless big line above it. And then this morning it even went between the last big line and the red, but never touched the red and didn’t stay high for very long. It’s fine on the interstate, but when I’m stopping and going in the city it keeps going high, cooling off, etc.

I’ve had my low coolant light come on in the past and I added coolant to it and cleaned out the system, but it’s not on now. No lights are on and the car is acting fine other than the temperature gauge. It’s been a REALLY hot summer where I’m at, but I don’t know if that makes a difference. I’m scared to drive it home after work today. What should I do?